They also have, or used to have, their own domains and servers, but those can be blocked at the (national) ISP level rather quickly.

On the other hand by mixing their content in centralized websites, they need to send takedown requests to every single one of them, since they would block legitimate traffic otherwise[1]. It's basically a workaround for IP and TLS SNI blocking.

Of course this is all to make it simple for non-techies to access their website / application. If they required using a VPN/Tor their adoption would probably plummet.

[1] Actually they blocked an IPFS gateway before so it's more like "well-known centralized websites".