Perhaps because one of the "three virtues of great programmers" is hubris - which Larry Wall optimistically defined as "The quality that makes you write (and maintain) programs that other people won't want to say bad things about." but which also/really means "Excessive pride or self-confidence."
I suspect many/most coders think a "personal trainer for coding" is a great idea - for other people (especially their idiot co workers). But they obviously don't need one themselves, and would never need to pay for new...