`__attribute__((packed))` wouldn't help here since the issue is about Lua's array/hash hybrid table design and memory allocation strategy, not C struct padding.

But it did help in the other way, in my reading of the paper [1]. So the OP is asking why this is not even an option on supported environments, and I too think that this is indeed a good question to ask.

[1] "Hugo Gualandi reported that just adding the gcc attribute __attribute__((packed)) to the definition of the structure TValue reduces its size from 16 to 9 bytes, without any sensible difference in performance."