Because residential streets have people and kids on them. Lower speeds greatly increase the odds of survival in car collisions with pedestrians. Plus it just makes everything feel calmer and safer, especially for cyclists and pedestrians. Who doesn’t want more walkable neighborhoods? If you’ve ever lived anywhere with sane residential speed limits, this would be a no brainer.

And being able to go 25mph vs 20mph saves very little time on residential streets - you’re not traveling 10s of miles on them on any given trip. If you’re going 5 miles for example, that’s 12 vs 15 minutes, assuming you’re going exactly that speed the whole time, which you’re not, because there are stop signs and roundabouts everywhere. So it becomes pretty negligible.