There’s the idea that he believes he’s entitled to a second term. And there’s so much he wants to do and it must be done as soon as possible. If any of that is even vaguely true, he won’t drop out.
(The fact he’s not getting any younger might be in there somewhere, but he probably doesn’t think about that at all if he can help it.)
“I’ve said some stupid things and some wrong things, but not that." – an actual Bill Gates quote referring to the 640k quote that won’t die.
But yes, it was probably satirically ascribed to him because of MS-DOS not having the capability to deal with any more than that amount of RAM for a lot longer than it probably should have.
The “temporary” solution of requiring an extra driver to be able to do so (EMM386.SYS or similar) remained in place right up until DOS-based Windows was allowed to die.
(The underlying reason was almost certainly ancient IBM PC memory-mapped IO standards, so maybe we could ascribe the original quote an engineer working there some time around 1980.)