What would happen if you had an upper limit on the amount of people you could follow? Modern social networking algorithms are set up such that you'll only see about the same 50-100 people on your feed anyway. Might as well see everyone you actually commit to following.
Limiting followings ensures that users aren't stuck in an endless stream of posts meant to keep them scrolling (see Build for Efficiency in Principles). They can enjoy their time on the app and then leave to go about their lives until the next day.
When followings are limited content creators on the platform have to compete to get your follow since they are scarce. This augments the quality of content on the app (see Cutting Fat in Principles).