Just wanna say. That's not NECESSARILY true. 14.7 is stoichiometry air fuel ratio. That's the ratio where, with proper combustion, all the fuel will burn all the air. It is a "complete" combustion. Any leaner. And there is not enough fuel to burn all the air. Any richer and you will be "wasting" fuel because there's more fuel than is needed to burn the present air.
But that doesn't make 14.7 "ideal". You can go leaner on cruise. As others have mentioned. Even though you aren't burning all the air. That isn't necessary. Especially at low load cruise. It really only takes something like less than 75 horsepower to keep a vehicle moving at 65 mph at low load. Don't need complete combustion for that. And you actually make MORE power on richer A/F ratios. Most guys with EFI setups start the tune between 12:1-13:1 and end up around 12.8-13.2 at WOT. Generally that's where you'll land for best power. Although you could be richer or leaner from that. It depends a lot on your engine and how well it completes the combustion cycle. They will run the car at various A/F ratios until they make a best pass or the most power. Oddly enough guys have tuned for best power on the dyno and ended up leaner than before. Left it at that A/F ratio and taken it to the track and lost time than their richer ratio. So it really does depend quite heavily on the setup.
