Many games do allow modding and Lua, a popular language for that, is a good starter language that's a lot like JavaScript. Mod.io[0] is good place to find games that use their excellent modding API which makes it easy for programmers to create mods, easy for the studio to distribute them, and easy for players to install them from within the game.
I say this as an experienced programmer who never modded a game before and was induced to write one [1] by this capability. It also made me learn Lua, and I'm glad I did!
To more directly answer your question, it's a lot of work for developers to permit any kind of UGC (user-generated content) and modding definitely falls in this category. One reason Lua is so popular as a modding language is that it creates an absolute sandbox and can even individually sandbox functions and language elements, making it ideal for allowing users to code against some aspects of the game but not all of them -- and not giving access to any of the game's code itself. These are always factors if you allow any user code.
I say this as an experienced programmer who never modded a game before and was induced to write one [1] by this capability. It also made me learn Lua, and I'm glad I did!
To more directly answer your question, it's a lot of work for developers to permit any kind of UGC (user-generated content) and modding definitely falls in this category. One reason Lua is so popular as a modding language is that it creates an absolute sandbox and can even individually sandbox functions and language elements, making it ideal for allowing users to code against some aspects of the game but not all of them -- and not giving access to any of the game's code itself. These are always factors if you allow any user code.
[0] https://mod.io/g
[1] https://mod.io/g/foundation/m/barrel-ofish