They have to install that somewhere and configure it to use that instance by default.

It’s not super complicated but also not trivial.

Edit: It’s not about “supporting” a frontend. It’s about “offering” them.

It’s usually just an extra container and a little vhost config. There’s very little to it, hence why lemmy.ca has all of them as options.

Because people have different tastes. Be glad you have choices.

Depends on the front-end from what I heard

It’s more code that can be abused. Balancing availability and security is never a straightforward task.