Ask yourself why Brady chose the Bucs over, the 9-7 Titans, the 8-8 Bears, or the 7-9 Jets, Broncos, or Colts? Those are the teams that finished as good or better than the Bucs and didn't have a $25M QB on the books.
The Bucs defense won the SB last year. They completely shut down Mahomes.
They were among the more injured teams, but I don't think the way it shook out was as devastating to their season as you make it out to be. They have so many weapons and you can have 2 out at a time, and be fine. They have so many defensive playmakers, you can have 2 out at a time and still be fine.
They finished 5th in points surrendered, for crying out loud. That's damn impressive for a skeleton of a defense.