Because Notepad or Calc aren't even virtually as complex as avast!. And since it's a part of the OS, they basically compile it together with the rest of the Windows core logic. avast! on the other hand has been built on 32bit code. In theory, they could make it native 64 but would require a lot of work. the question here is, is it worth it? And the answer is, it's not worth it since you wouldn't really gain anything particularly measurable. But the parts that do mattter, the scan engine, drivers, core logics, that works on both, 32bit and 64bit levels.
Eventually, when 64bit completelly replaces 32bit (like it happened in teh past when 32bit replaced all of the 16bit code in Windows Vista 64bit and above), then avast! willa lso get a nudge to a full 64bit.