The truth is that even in his prime Dez was a relatively limited player who won by overpowering and out-jumping/reaching corners. As soon as he slowed down physically he went from mostly winning contested catches to mostly losing them, and stopped breaking off short catches into long gains. What's left is a guy with a low catch rate
compared to the other Cowboys receivers (!!!) who also doesn't make big plays.
A team like the Patriots looks at this and sees literally no upside. They can manufacture yards for guys with limited athletic ability, but only if they can do the following:
- Master a complex playbook and a completely different play calling system from most teams
- Run precise routes that put them in exactly the right place at exactly the right time
- Be reliable
Dez hasn't been reliable since 2014 and was never a technician. The Pats could probably find a role for him, but from their point of view he's just a worse Chris Hogan now. They already have dudes who can't separate, and at least the guys they already have know the playbook and will go where Brady tells them to go.
The rules are different if you're a physical freak, they didn't make Randy Moss run a ton of different routes, they let him take the top off the defense and maybe run the odd slant and score a million points that way. Early in his career Gronk didn't instantly master the whole offense, but that was fine because he could overpower literally everyone that tried to cover him. That's why Josh Gordon was worth taking a (cheap) gamble on.