There's a lot of explanations that don't involve the DSM. People naturally form parasocial relationships with media characters--books, cartoons, comics, games--anything. Our emotional systems can still respond authentically despite our knowing it isn't real. Most of us handle it just fine and not bleed into the sorts of things you're referencing.
Some individuals have had other things go on in their life where such relationships feel "safe". The character won't harm them or reject them. People with trauma, anxiety, and just trouble nagivating the social landscape and it's not because they're broken--they're just finding what works for them. It still doesn't need to rise to what some would say unhealthy, disturbing or creepy.
Not all people with waifus completely isolate themself. It's just a thing for them and they maintain normal relationships outside of things. It's just easy to focus on the "weird ones" because social media often brings them to the fore front. That's what we see, so it gets stereotyped as such.
On the autism take: I would have like to get ahead of this, but I arrive late. Armchair-diagnosing strangers based on a single behavior you find unusual is both scientifically meaningless and genuinely harmful--even if you share the diagnosis (like myself and all of us autists here are unique from each other). Yes, there's research showing that some autistic people can form strong connections to fictional characters—but correlation isn't causation, and more importantly, that research doesn't say what a lot of people think it says. Autistic people aren't a monolith, and neurotypical people engage in parasocial bonding constantly—sports fans, celebrity crushes, fictional OTPs, the works. Using "autistic" as shorthand for "weird thing I don't relate to or understand" isn't insight, it's a lazy dismissal dressed up as explanation. If you actually care about understanding why people form these attachments, psychological labels applied to strangers won't get you there.
My take: Some people really do prefer fictional intimacy over real relationships—that's valid if it doesn't involve harm, and leaves room for growth. Others treat it as a hobby or appreciation without deeper attachment. Both are normal variations.
We've seen this cycle before. In the 1970s-80s, psychologists and commentators warned about women becoming emotionally dependent on romance novel heroes—"fictional men" who offered perfect devotion without the messiness of real relationships. Today, romance novels are mainstream entertainment, and that specific moral panic faded. Similar concerns came up for online pen pals, grown adults still playing videogames, and even comic book collecting in earlier years. Each time, something that looked strange to outsiders turned out to be a normal variation of how people find connection and comfort.
