Why not?
I’ve been in situations where I couldn’t release the code to a project, but I was able to use some decent libraries because they were MIT licensed.
So I’m happy to do the same for libraries I write so that others in similar situations could also receive the same benefit I did.
I see it as an act of public goodwill, like paying it forward for the times you can’t directly contribute to another project.

Just my personal view on it, anyway.
I’m not claiming it’s a bulletproof solution or that it isn’t open to being ‘abused’.