Why is therefore not written therefor?
The "fore" in therefore does not mean "for". It means
- situated or placed in front
- the front part of something
That same root is used in words like
- before, aforementioned, foreboding, etc.
I don't suppose you are proposing to change the spelling of all these words?
The word "therefor" does use "for" and means "for that".
P.S. When you thing about it, the German cognate of "fore" is "vor", and the one for "for" is "für".