I think under windows x64, it still uses user32.dll and a bunch of other 32 bit libraries. Why there is no user64.dll?
C:\Windows\System32\user32.dll is a 64-bit library.
The 32-bit version is in C:\Windows\SysWOW64\user32.dll.
They can't change any names because that would break all sorts of programs with hard-coded paths.
The names "user32" and "system32" and so on existed long before the modern 64-bit editions of Windows. The names were intended to distinguish them from the 16-bit versions, not from the 64-bit versions. On 64-bit Windows, it's still not 16-bit, so it's still "system32", "user32", and so on.
Nobody planned it that way, it just happened.
来源：https://stackoverflow.com/questions/9169409/why-windows-64-still-makes-use-of-user32-dll-etc