Because self hosting is a big commitment, costly in time and resources, and just generally a skill that the vast majority of users simply don't have (and they'd be well advised not to bother). There are a few managed mastodon services for this that I've been eyeing for corporate usage as I can see the moment coming that we'd might to be on mastodon at least as Twitter/X has been imploding a bit lately. I could probably figure out self hosting but I have more urgent things to do.