The reason why volatile variables aren't extended to 64 bit is because in embedded and systems programming, it is often used to access memory mapped IO register

The reason why volatile variables aren't extended to 64 bit is because in embedded and systems programming, it is often used to access memory mapped IO registers - so any sign extension or truncation might lead to a wrong value being sent over the bus. GCC also has a keyword (register) to make sure that the variable is sized exactly right and fits in a register (iirc, there was a way to even ask the compiler for a specific register to be used for the variable in case of inline assembly being used).