It's been double-stable since early 2025 (version 3.2), so it's weird that Ubuntu doesn't have it when even Debian does. I've seen a couple projects holding back on the upgrade because of that.
With the way the software ecosystem works now, a lot of projects just include a copy or submodule because it's self-contained and a good candidate for it. It dynamically loads most dependencies, so it's pretty light-weight, considering. The whole static linking licensing problem is gone now, too.
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".