Probably for the convenience of partners/children.

Lots of people who aren't super deep into computer nerdy shit get outright confused when faced with options they've never heard of like "Plex." Sometimes it's just more convenient to respect your partner and keep Netflix because it makes sense to them.

Plex doesn't have easy-to-use parental control options and a lot of parents who might have the time to set up a Plex server for themselves may not have the time to muddle through the parental control options than just accepting that most streaming services have a kid-friendly section already set up and easy to access. Plex's matching algorithm isn't perfect, and opening it to your kids means you've got to make sure the matching is perfect. I've definitely had adult shows be recognized by the matching algorithm as children's shows. If you don't catch that, your kid can end up watching something you'd rather them not. It's just a lot more work to manage your Plex parental controls than just using the off the shelf setup from the streaming services.

Neither of these apply to me, but I know folks who have Plex plus subscribed services for these reasons.