It doesn't help when you have the lowest percentage of catchable passes in the NFL in the last 2 years.
When Dez runs the route incorrectly, it makes a perfect pass off target.
A QB is normally coached to just look at each receiver in his reads for a split second. Coaches constantly work on tempo with them like a band instructor with his students, especially drummers. The Cowboys had Dak stay with Dez extra long (About 3x) in an attempt to see of he could get open. That caused other problems but it was the team and Dak going All-In to give Dez more chances.
When Dez was not open in those situations, Dak usually had to get rid of the ball and would basically throw it away but would still throw to a safe spot where Dez might be able to get it but the CB could not.
For instance when the CB wss over (behind) Dez, Dak might throw it short. If Dez could react quickly there was a chance he could catch it but there was no chance the CB could go through Dez to get to the ball.
The opposite example was also true. If the CB undercut Dez near the sideline, Dak would throw high. If Dez was able to time his jump he had a small chance to catch it.
In other words Dak was throwing the ball away but was still giving Dez about a small chance to catch it.
Dak didn't really do that with other receivers.
The bottom line is that complained to just throw him the ball but he just couldn't get open despite them giving him about a 3x time advantage in Dak read progression.