The Steam client is super old and crusty. Ubuntu tried to remove 32 bit libraries like Apple a while ago but Valve told them if they did that, Steam would no longer be supported so the plan was scrapped.
Even if Valve finally get around to updating the client to 64-bit (it already is on macOS AFAIK), most old games are not going to be updated so insisting on keeping the 32-bit libraries around is the right thing to do. Keep in mind that the distro just needs to supply the base system (libc, OpenGL drivers, etc.) in 32-bit and Steam already comes with copies of everything else via the steam runtime.
