You are working wrong.
You can’t work wrong and compain that stuff doesn’t work or “isn’t compatible”.
Your rig and animations are your rig and animations.
Unless you go out of your way, stuff you buy isn’t magically compatible with other stuff. Even when you do find something where the skeleton is the same, they are usually poorly compatible at best.
If you spend time to make whatever animation you buy compatible with your rig, then thats a different story. And one way to do it is the retargeting links that were shared above.
What you are doing wrong is that you expect the engine’s sorry excuse for a rig to be compatible with everything. Its not.
Most animations you buy don’t even have the approrpriate skeletal markers needed to display what they were meant to work for correctly.
Its up to you - and only you - to make your own decent skeleton/rig/character and compile the animations from whatever you want into it so that things work for you.
If you start using metahumans or other similar shortcuts (including retargeting if used at runtime), then you are both doing it wrong, and compromising your project.
As far as why there cannot be just one univerals rig - thats because when you try to you end up with s*it like the metahumans.