This is a huge pet peeve of mine. Bounded accuracy was designed for combat, not skills. Expertise exists because bounded accuracy, using the same scale as combat, would ruin the skill system.
In combat, power scales along multiple axes: Attack bonus, AC, hit points, number of attacks, damage per attack. Because of this, you can put tight constraints on attack bonus and AC -- you could eliminate them altogether if you really wanted to -- and still have a wide range of combat power. A level 20 fighter is more formidable than half a dozen level 1 fighters, even if you boost the level 1 fighters' attack bonus and AC to match the level 20.
But with a skill check, you have your bonus and the DC and that's it, full stop. Using only proficiency + stat, the highest possible skill bonus is so low that even a highly skilled character will frequently fail at basic tasks. There has to be some other factor to make up the difference. Expertise is a hack fix, but it's a whole lot better than no fix at all.