This sounds like an honest question, so I'll offer my perspective. Directly from the wiki:
"Social media introduces additional opportunities for parasocial relationships to intensify because it provides more opportunities for intimate, reciprocal, and frequent interactions between the user and persona."
I think that's what enables a bad form of parasocial. Compared to TV personalities, we actually CAN have a (limited) form of reciprocal interaction with the waifus. And for most normal fans, that's awesome, engaging, and fun for all parties involved. But as with most things in life, too much of a good thing can become a bad thing. And we're on 4chan, with enough autists that overdo it and overestimate how close they are to the waifus and assume they know them as fully-realized people.
So enough people overdo it that parasocial is often used as a pejorative here. But you're right that parasocial is not inherently bad, and it offers a lot of positive things when used "correctly"/in moderation!