They won’t simply because - as artists - they’ll never agree to a standard made up by person X of nowhere at all.
For the most part, they can’t even be bothered to follow data sheets and studies that are presented to everyone by Game Gems, so there is little to no point in even suggesting this.
Besides, Without someone on a team who is capable of making adjustments (which take barely a week to learn from 0 knowledge) even with standards you’ll still have issues.
Not everything will work flawlessly.
Not every weight paint will be the same between assets (And you really cannot standardize the paint. It either works for your use case or does not).
Not every mesh will be free of clipping.
not everry mesh will have the correct morphs assigned for your case.
etc.
Again, things are never that modular. You always have that 1 off item that doesn’t fit into the rules.
the fact you have rules at all, will threfore discourage progress, innovation, etc. Making it a constant hassle to find a workaround that let’s you do what you need. (Kinda like engine changes? Hurray!)
There’s also the fact that imposing any rule on art is just wrong…
But we aren’t talking philosophically here. Making humanoid characters for use in a videogame or movie or3d render is something that follows some rules already.
What you are talking about however is tantamount to censorship.
Epic and the Marketplace curators would never go for adopting such rules.
Something like daz may - perhaps even successfully.
(Its their biz model currently isn’t it?).
PS: for finger retargeting to look a little better you need to set the root of the finger bone to follow scaled animation rather than skeleton. The rest of the chain should then be free to wrap up/curl as needed.
if not, you can use corrective poses on the hands that alter the base animation for the specific pose. (And now a days you can make those in engine with control rig).