You can create elaborate class systems using metatables in Lua; the problem is that there are hundreds of them. I've done some Lua programming using Love2D, and one of the painful parts was that a lot of the major libraries implemented different class systems (or rolled one of their own), making it hard to include in a single codebase. (At least Lisp has CLOS as its common class system...) Also you can mitigate the "undeclared variable = nil" problem using a script like strict.lua (https://github.com/deepmind/strict/blob/master/strict.lua), but when you try to expand that for field accesses you then want to make sure all of those libraries with different class systems or methodologies are also compatible with it (which a lot of the libraries do not).

The underlying problem with Lua is that it went too wild with the philosophy of "being the most expressive using the least amount of features" (so you can implement everything using only metatables). Although many would say that this is a good thing, for me it seems better for a language to have some kind of subset for which people can conform and rely on.