Nothing is stopping apps from shipping with 32 bit libraries if they need them. This isn't like what happened with ARM where processors dropped support for 32 bit code.
libGL/libVK for OpenGL and Vulkan support is a big one, you need that to be fitting the exact version of your kernel Nvidia driver if you are using Nvidia cards. Of course all other vendors play nice with Open Source and Mesa libs, but especially for gaming, a lot of people unfortunately still buy Nvidia. Games cannot just ship their own 32bit libGL for reasons of license and versioning.
On Windows, this is like telling game developers that they need to ship all the graphics drivers for their games. Let's skip the hassle for the developers' side. For the users, not only would that be a situation where you could no longer update your own drivers to fix problems, it would be a compatibility nightmare and likely introduce new problems because of interactions with the system drivers and kernel level.
1. They need to also keep the multi arch support enabled in their kernels 2. Apps such as games and steam depend on Mesa, and you don't really want to ship Mesa
A good compromise would be for Valve to support Flatpak for Steam, but if they can't be bothered to build a 64 bit Steam they sure don't want to bother with that