As a GM and a player, there are a few reasons I see as to why:
Risk vs. Reward
For the longest time most Deckers would do their job from a place of safety. There wasn't too much risk in their job: sure, they might get a little brain fry, but that's only if they're going after the really big or shady hosts. In many cases the worst the decker would have to deal with is repairing or buying a new deck. Street Sams can spend as much money on their chrome, but they also risk catching a lucky bullet. Magicians get to wiggle their fingers, but...are more likely to catch the lucky bullet once the others figure out who's flinging the mojo.
Deckers, though, can hide in a safe place and take out every single wireless item the opposition has. They can hack into hosts and make life hell for the rent-a-cops in the Arcology they're running in. Ultimately, Deckers tend to be the least likely to end up with bullet wounds.
A good Decker doesn't often go long without having an opportunity to score some of that sweet cred. Any good team wants them around to make sure they get the most use of their chrome and gadgets without having Mr. Corporate Spider taking them down. Additionally, if that Decker is poking in a host, they're likely able to steal some paydata to help boost the team's profit...or their own.
Decker vs. Technomancer
A Technomancer is always hot simmed. A Technomancer has a higher chance of dying from working in the Matrix. If you brick a Technomancer...well, let's just say you don't want to use them as a paperweight for too long. Hard to get rid of the smell. Deckers can deal with some drek, too, but in many cases have ways to mitigate having anything but that weird smell after a bank run went a little south. Technomancers have to have good attributes, since they are their decks; a Decker just has to know how to use a deck.
Ultimately, it's all they need
Sure, 123k might seem a lot. But all you need after that are some trodes and you're ready to go. Ready to go for quite a long time. No need to maintain an arsenal, keep up your armor, get your limbs replaced, or be born with the "special" stuff. All you need is the desire to code and the nuyen (or resources) to buy or cobble together a thing someone might call a deck. Sure, you're going to want to upgrade to take down those heavier hosts, but after you get that deck all the scratch you make only needs to go to keeping you fed and sheltered.
Aside: I can't find RAW for making a deck, but it's a thing that has been discussed with me by others. Data Trails does have a section for modding a deck, though, which you could use as a GM if you wanted them to be able to make one.
Data Trails offers a cyberdeck for as little as 21k if the Erika is too high for you, and some specialty decks that are cheaper than their peers but can't be reconfigured like them.
Also, while doing the research for the question in the comments, I came across another point: A Decker can reconfigure their strengths and weaknesses on the fly with the standard decks. Technomancers are left with what they have, but a hot Decker can shift their deck around as needed.