I think the main reason Hive doesn't collect that information is because it has no need to. The main reason other platforms ask your age, sex, ethnicity, etc. is because they are selling your information to advertisers for a profit. Collecting demographic information and selling it is basically the whole business model of Facebook and The Others.
Hive does not sell your information to anyone, so they have no use for it. Demographic information is completely irrelevant to you interactions with the blockchain. It doesn't forbid the sharing of that information. You can put it in your profile, if you want, and many people do.
I can't say for sure what the developers were (or are) thinking as they build the platform, but it seems likely that privacy and de-commercialization were among their concerns.