Why can't HttpService call Roblox's own web APIs?

Roblox blocks HttpService from calling its own domains on purpose, so you can't hit their web APIs directly from a game server. The usual way around it is a proxy that forwards the request for you. There are public ones but you're trusting a third party with it, so for anything serious people host their own. Also check there isn't an in-engine method that already does what you want. A lot of the common data (groups, user info, avatars) can be pulled without HttpService at all.