Ubuntu does include it, since 24.10, but Ubuntu 24.04 doesn't. It just barely predates (an ABI stable release of) SDL3. Now that 26.04 is out, it won't take too much longer for that reason to hold off on SDL3 to disappear, but I think we'll at least have to wait until Pop!_OS rebases before it is a completely done deal.

And yeah, in the meantime, I have just been using CMake's FetchContent system for SDL3. With improvements on both CMake's end and SDL's own build system, it is quite seamless. I still wish it was a bit more elegant but I'll take "easy".