Because it's being assumed that if you don't have LinkedIn then you are not a suitable candidate to hire, sometimes in pre-screening they explicitly ask to provide it, or get screened out. So when you make something like that a de facto requirement to get a job, or at least to be considered, then people will complain about it. Per your analogy, it's like I have to have a mandatory strip club membership to be considered for xyz.

My issue with LinkedIn goes beyond the cringeworthy content. It's the best platform to stalk people and collect any info using OSINT. Unlike other platforms where you can have some nicknames etc., you are most likely to have legit info in there. I wouldn't be surprised later if digital ID in smartphones will be required to update/sign up there, "to make sure we fight fake accounts!!"