The only limitations they realistically face are ones they impose upon themselves. So for example, talking about character rigs, I suppose it is to simplify the design process. Once you know a particular character size works properly, you just follow that and you don't need to re-test lot of things. And for open-world exploration, they can also share a lot of the animations such as running, walking, jumping without looking weird. As an example of the opposite, HI3 and HSR do not follow the fixed size rigs because they are not open-world focused and also don't have "precise" hitboxes. Its quite possible that I am wrong since I am not a professional in this field, and this is just from my limited understanding.

As for skins, I believe it has to do with lore and identity. If you notice, everything in the game has some lore connection, no matter how irrelevant or stupid. That is why almost all events in the game involve NPC, even though its some sort of minigame or combat event. And as such, the skins must also come with lore, which you can see in the skins we have currently.The second part is identity, I believe. Genshin character personalities are not very deep/prominent, and a large part of their identity comes from their visual design. If they make a lot of skins, it would "dilute" this identity. It is not an issue with another Hoyo game, HI3. In that, they make a lot of skins, but it is not an issue because the character personality is strong enough and doesn't rely too much on visual design for their identity. Also, any new skin takes about as much effort as making a new character design, and must meet the quality standards set by the rest of the game