Because the problem they've chosen to solve is "secure reliable e2e encrypted messaging via the internet". (or something very similar to that)
It's a big enough problem on its own. Tacking "p2p" on as well makes it a different thing, which is not what Signal are doing.
I want my Signal messages to arrive at my friends Signal clients, even if that's later after they've recharged their dead phone. I don't want my Signal messages to vanish if I (or a friend) is on a WiFi network where p2p is blocked. There's a serious need for a centralised server for Signals goals to be achieved, so they have one. The fundamental design then means the added benefit of _also_ implementing p2p where possible is small, while the effort/cost is high. Of course that didn't build that.
It's a big enough problem on its own. Tacking "p2p" on as well makes it a different thing, which is not what Signal are doing.
I want my Signal messages to arrive at my friends Signal clients, even if that's later after they've recharged their dead phone. I don't want my Signal messages to vanish if I (or a friend) is on a WiFi network where p2p is blocked. There's a serious need for a centralised server for Signals goals to be achieved, so they have one. The fundamental design then means the added benefit of _also_ implementing p2p where possible is small, while the effort/cost is high. Of course that didn't build that.