The largest NFL stadium seats about 85,000 people. So you can't sell more than that for the single game. Two years ago they actually set the record lowest attendance due to Covid restrictions - only about 21k were allowed in the building. I believe it's the first SB to ever sell below capacity, and you'd have to go back to the second world war to find an NFL championship that did the same.
Also, music is fundamentally better live. Football is not. I say this as an avid fan of both who attends both live.
There is always the option to change the odds for fans. E.g. offer cheap tickets for members of the fan club and last minute tickets to fans who post the most convincing appeals on social media.
And I doubt that a very large number of fans are concertgoers at all. How many football fans actually go to the superbowl?
I looked it up and it's ~80k, while 100M watch on TV. The cheapest tickets are ~$5000 dollars