He already tried that twice.
I think the thing that over-mystifies Frank and John's relationship is the "money sink" notion. There's this idea that John is burning all of Frank's money on his stupid hobbies, but really, the stupid hobbies are as much or more Frank's as they are John's. The campaign burned cash, but who was actually more interested in the campaign? Who dressed up in the Godzilla suit, did the arts and crafts, appeared in all the ads, and wrote everything that wasn't done through an Ambien haze?
Same with the cars. Who's actually doing all the work? If Frank didn't enjoy this stuff he'd just send the missus off to whatever the Massachusetts equivalent of "Pimp My Ride" is, hand him the credit card, and say "Go nuts". But instead he's out there spray-painting the car on his lawn.
Really, I think they're just a couple of manchildren with similar tastes.