Bill Belichick underestimated how well Tom Brady would age is the main reason. Given the rest of the roster, he didn't think Brady was worthy of the all in treatment what he figured would be the last few years of his career.
I believe Bill Belichick was surprised at how well Brady did at Tampa. He probably thought Brady wouldn't do as well on the deep throws but there is a difference between throwing the ball deep to Evans/Godwin vs what the Patriots had in 2019-20 and even the year before that.
For sure if Brady had stayed and the opt outs didn't happen last year, Patriots wouldn't have this much cap space. But there is almost always a way to create cap space. The question is if you are thinking medium to long term which Bill Belichick is also paid to do is whether or not it is worth it.
It is okay to admit that Bill Belichick with 20/20 hindsight may have made the wrong decision with Brady. More of Belichick's decisions have been very good than bad.