Upon re-reading the PHB inspired by David Coffron's answer and Phil Boncer's answer, I think I actually found the answer to this question. The numbers as written fit perfectly in the PHB without any hand-waving at all. The confusion comes from different uses of the word "laborer" in different places. Essentially, based on the text, there seem to be three tiers of people you could conceivably call "laborers". All quotes come from the Player's Handbook pages 157-159 unless otherwise noted.
Untrained, day-job laborers
Wage: These laborers are making 1sp for a day's labor (PHB 143), doing things that require no real training or skill. They also don't have a set, day-to-day job (which would pay more). These could be analogous of the real-life immigrant workers David Coffron mentioned, or they could be someone who has suffered a setback.
Lifestyle: By default, these people would be living a Squalid lifestyle (1sp/day):
You live in a leaky stable, a mud-floored hut just outside town, or a vermin-infested boarding house in the worst part of town. You have shelter from the elements, but you live in a desperate and often violent environment, in places rife with disease, hunger, and misfortune. You are beneath the notice of most people, and you have few legal protections. Most people at this lifestyle level have suffered some terrible setback. They might be disturbed, marked as exiles, or suffer from disease.
This would fit someone who isn't able to find a job because they were fired or injured or fell sick. These people are poor and desperate, and would do basically anything for 1sp/day, because the alternative is being knocked down to a "Wretched" lifestyle (you don't have to read the PHB to know that's bad).
However, there are several possible ways they could be living at or near a Poor lifestyle (2sp per day) without long-term employment:
- They could be living in "Poor" conditions, but with twice as many people. This would lead to better, if more cramped, conditions.
- They could be essentially "working two jobs". If they get up early and go to bed late, it's conceivable that someone particularly industrious could keep themselves out of Squalid conditions (similar to the people in real life forced to work 80 hours a week to get by).
- They could have something else on the side - perhaps they also rob houses at night, or help smuggle, or work as a loader for the carpenter for 5cp a night.
Untrained Laborers with jobs
Wage: These people would be paid the 2sp/day cost shown in the "Untrained hireling" section of the services table. If you read the section on services, it tends to describe people with more of a set job and long-term employment. It would make sense that someone with a long-term job would get paid more (this would be equivalent to the $15/hour figure David Coffron quoted.
Lifestyle: With a 2sp/day wage, these people are living exactly at the Poor lifestyle:
Poor. A poor lifestyle means going without the comforts available in a stable community. Simple food and lodgings, threadbare clothing, and unpredictable conditions result in a sufficient, though probably unpleasant, experience. Your accommodations might be a room in a flophouse or in the common room above a tavern. You benefit from some legal protections, but you still have to contend with violence, crime, and disease. People at this lifestyle level tend to be unskilled laborers, costermongers, peddlers, thieves, mercenaries, and other disreputable types.
Had I noticed the bolded section of that quote the first time through, I may not have posted this question. It makes it much more clear. The numbers match up more easily for these people, and it seems a thematic fit as well. Unskilled laborers with steady jobs would still have "simple food and threadbare clothing", but they wouldn't necessarily live in the vermin-infested slums.
Untrained laborers with jobs at the castle
Wage: I believe the wage for these laborers would still be the same (around 2sp/day). However, the Player's Handbook says the following under the services section:
If a high-level adventurer establishes a stronghold of some kind, he or she might hire a whole staff of servants and agents to run the place, from a castellan or steward to menial laborers to keep the stables clean. These hirelings often enjoy a long-term contract that includes a place to live within the stronghold as part of the offered compensation.
Lifestyle: I believe these are the laborers mentioned under a "Modest" lifestyle:
A modest lifestyle keeps you out of the slums and ensures that you can maintain your equipment. You live in an older part of town, renting a room in a boarding house, inn, or temple. You don't go hungry or thirsty, and your living conditions are clean, if simple. Ordinary people living modest lifestyles include soldiers with families, laborers, students, priests, hedge wizards, and the like.
These people might only be making a few silver a day, but as part of the arrangement they are able to live in the servant's quarters of a castle. It would also be reasonable to assume that, at least at some castles, food is provided. Their living conditions in this situation are much, much better than anything they'd be able to afford with their own wages. They're still living in "simple" conditions, but they are clean and safe, and they are "kept out of the slums".
This also explains why someone would want to come work at your keep - the wages are the same, but they get a much nicer and safer place to live inside your castle. Those living conditions are accounted for in the cost of constructing a such a facility (e.g. 50,000gp for a small castle). Without this difference, they wouldn't have any real incentive to leave their friends and family to come work for you at the same wage.
Beyond Untrained laborers
Beyond untrained laborers, the numbers actually make pretty good sense. It costs 2gp/day to hire a skilled hireling, the Comfortable lifestyle costs 2gp/day, and "skilled tradespeople" are listed among the people you find living a Comfortable lifestyle. Beyond skilled tradespeople, we've left the common folk and are looking at the upper class of merchants and nobility.