Where is this ratings proof? Link please.... Put a game on and football fans will watch it. Not sure I understand your point about scheduling. I don't see how it matters what non-Sunday day a team plays on.
Prime Time games aren't about attracting "football fans," it's about garnering the casual football fan. That's how the networks get bigger ratings.
Historically speaking, Friday and Saturday nights are the worst nights of the week ratings wise accross all genres (sports, sitcom, drama, reality, etc.). Thursday, Sunday, and Monday are bigger nights ratings wise because those nights historically have more viewers.
TV programmers don't set out to create new viewers to TV. They want to get the most of the pool of viewers that are already there. Friday and Saturday night have the smaller pool of viewers than the other big nights I mentioned. If the NFL had a game Saturday night they would only be sharing the viewers with College football and both programs' viewership would suffer. For Friday, although there is no big sports competition, the viewer pool would be smaller and the ratings would be lower than on other nights.