Except they have been working on it, by repackaging apps to strip out unneeded assets.
And it is in Apple's interest to have smaller apps, because then people will be able to download more apps. They'll be willing to try more apps, because there will be less of a barrier between seeing the download button and being up and running.
Plus Apple likes shipping lower end/cheaper devices to people who won't buy the high end.
When your low end configuration is hard to use because Facebook takes up 29% of the storage... that's a problem. Customers get mad.
When they can't update Facebook because it wants more space and the device is full and now the user is locked out from their friends... customers get mad.
When updating a handful of apps uses up 70% of their monthly data... users get mad.
And it is in Apple's interest to have smaller apps, because then people will be able to download more apps. They'll be willing to try more apps, because there will be less of a barrier between seeing the download button and being up and running.