Because coaches and players are still playing for their jobs... Sure losing worked well for the Colts, but there was a massive house-cleaning in the offseason, and many of the coaches and players aren't even around anymore to reap the benefits... I mean, maybe if the owner colluded with everyone to tell them their jobs were safe, but even that is pretty ridiculous when you have actual personnel evaluations to make in the offseason. I know if I were an owner or GM, I'd want to determine first and foremost who I'm getting 100% out of, and who can be replaced.
Also, if fans who are paying good money to go see their team play even suspect they're tanking, that only stands to lose you revenue too. Even in a futile season, there are certain games on the schedule that are like your Superbowl if you win.
The same could be said in the NBA as well, though. Teams openly tank all the time in the NBA for good draft picks. Just ask the Bulls and Blazers the year Michael Jordan was drafted or the Golden State Warriors last year. I don't get how football is somehow different. I would rather go 2-14 than 8-8 and miss the playoffs. If I were an owner and my team was absolutely terrible, I would tank on purpose for a couple of years, acquire a ton of great, cheap talent, and then compete. It's better than fighting tooth and nail to be 6-10.