It's the same thing as making WINE: you implement the ABI in userspace. It's a pain in the butt and it suffers from compatibility and performance issues. Programs run with WINE often run into problems not found on their native Windows environment. It's better to have a standard that each operating system implements so that apps don't need a weird portability layer in-between.

Sadly, Linux doesn't give a shit about other systems, so any effort to standardize containers will happen outside Linux, and Linux probably won't pick up its changes because they already have Linux containers. So a WINE like thing would be the most workable option.

The current state of affairs is actually a much better solution. Just make all containers OS-specific, and ship the OS kernel and dependencies along with a shim, and run them with QEMU. Apps run way more reliably this way with no porting required. The devs do less work, the users do less work, there's less bugs, and you can run any app in any OS.