Mundane traps are mostly for low-level encounters
This can easily be seen when you see that many mundane traps are capped at DC 20, while magical traps have a DC set as 25+spell level. Meaning that as long as the character can take-10, has the time to search for it, and has a total bonus of +24, which can be obtained without much effort by level 10th or so, they are good even against the most dangerous magical traps.
anyone skilled in Perception automatically succeed at finding them
They can reasonably find them most of the time, but not all the time. You can expect a character to take feats and abilities to improve their trap finding check, and even take-10 on many checks, but you cannot expect them to always succeed. Because there will be situations where they will have distractions (combat), there will be situations where they cannot wait and check every square in the room for traps (running away), and there will be situations where they overconfidence will cause their ruin (traps that are reset by an enemy).
Keep in mind that finding the trap does not disable the trap for safe passage. Characters who expect to be good against traps have to invest their abilities on two skills, not one. It is not weird to be good at something you want to be good at. If you wasted a feat taking Skill Focus on a skill, for instance, you should expect that character to be really good at it.
You are also incorrect if you assume that all mundane traps are capped at DC 20. To not go beyond traps that can be found on low-level adventures, here are a few CR 2 traps that have a DC higher than 20:
And here are some that are also CR 2 and lower than 20, meaning that even a non-specialized character could find them:
As you can see, DC 15 traps can be found by any character that has at least +5 on perception, or even those with a +3 that is receiving help from another character. Even among mundane traps, there are a few that go way higher than your average magical trap, like the Collapsing Stairs (DC 29) or Deathfall Pit Trap (DC 30).
Traps are not meant to be impossible to find, they are meant to take adventurers by surprise. Magical traps use runes and magical trickery to catch adventurers by surprise, which is why their DC is usually higher than mechanical traps.