Because they had a stadium that seats 76,000, which made it one of the largest in the NFL.
Wayne Weaver, the first owner of the team, wanted to build a stadium that seated 65,000 - which is the capacity now. But the city balked because they host the Florida-Georgia game, which requires a greater seating capacity.
The current stadium seats more than Chicago, more than Pittsburgh. According to your logic, shouldn’t they be selling more tickets too?
They were selling out before, then they stopped, now they’ve shut down over 10% of the stadium, and they’ve still don’t sellout, and still use an obscure NFL loophole to avoid blackouts. That is a problem selling tickets.