I publish lots of apps and don't have a great explanation either. I guess if you say that you just fixed some bugs, neither the app reviewers nor the users will pay too much attention and just approve/download the update, which is what you want as the app developer. Or it's just laziness. Or a communication disconnect between the person in charge of writing the changelog and the product/development team.

> I have mostly seen this with Google and Instagram.

Its because you are actually the product. They have no incentive to provide you with more information. They just want your eyeballs and keystrokes so they can sell your attention and interest to advertisers.