Not only it's unnecessary, it also bad GUI. In apps with infinite scrolling your position doesn't get its own state, so if you eg. go view a post & return to Instagram timeline it opens at the beginning. If you use media more thoughtfully as opposed to mindless scrolling, it just doesn't work.

With pagination you could say return to IG after a week, go through 5 pages of whoever you're following, check out a post, go back knowing you stopped at 5th, and continue from there. You can also set a limit (I'll check 10 pages tops and then go to sleep), which is helpful even with algorithmic feed.

I'm sure they can screw up pagination too if they wanted, but typical implementations don't have this issue