Because linkedin's value proposition is to make you look good (superficially) to future employers.

Nobody wants to interact on linkedin, they want to look impressive. This accomplishes that.

Exactly. Those churning out such posts on LinkedIn, would very much prefer if other people did not even carefully read the actual content, but rather simply assumed “Wow, this person is capable of generating a wall of text day in and day out, he/she must be a subject-matter expert and have great English skills”.