The compatibility layer mention is valid for many projects, and I personally greatly appreciate that it exists, even if it's a bit buggy

osu! however, is only officially distributed on Linux via AppImage (and semi-officially via Flatpak) and bundles its own SDL for the reasons that, to my knowledge, range from the general pinning to a known stable version approach to the team backporting functionality from SDL3 to SDL2 (last one I remember is non-text-based clipboard functionality, not entirely sure though)