Among many other things, privacy. If you know an age range that's more information about the user than you should have, and you can use it for marketing. And if you can ask for an age range, then in practice you know a birthday; just keep asking until the range changes and record when you saw it change.

It's also the wrong way around: apps shouldn't ask for an age range, apps should provide one and the OS should do the checking.

I also object to it because it's a foot in the door for demanding that websites and apps care about that age range.

And most importantly, devices already provide parental controls in practice, and this law doesn't make those any better, it just unnecessarily cements one aspect of them into law. You can, already, on many devices, say "don't allow installing apps without permission", or "don't allow installing apps except those with this rating".