Because Hive isn't a social media platform. It's a blogging platform.
That's always been one of the biggest problems with steem/hive (along with centralization of stake, the ninja mine, etc.)... people try to sell it as a social media - but it's just a way for bloggers to have less overhead, and potentially get some ad-free revenue.
Hive is not just/only a blogging platform. Hive is a blockchain with multiple platforms and many websites, from which many belongs to the social network category. For example Inji, DPoll, D.Buzz, 3Speak, etc. The last one for example is a video sharing website.