I don't think it's just because the devs get bored. It's because the users crack the shits if their specific use cases aren't handled, and when it comes to 'frameworks' nobody wants to switch or use anything that's not the most popular, so over time they just get more and more bloated trying to handle more use cases.
It's twice as bad if there's financial incentives for having a userbase (see all the tools not owned by FB/Google etc where the contributors sell training etc) because then it literally pays to add as much crap into your library as possible to try and get as many people using it as you can, regardless of the effect it has on quality. Even if something would make more sense as a fork or alternative, it's getting added in, whether it's good for the library and ecosystem or not.
It's twice as bad if there's financial incentives for having a userbase (see all the tools not owned by FB/Google etc where the contributors sell training etc) because then it literally pays to add as much crap into your library as possible to try and get as many people using it as you can, regardless of the effect it has on quality. Even if something would make more sense as a fork or alternative, it's getting added in, whether it's good for the library and ecosystem or not.