The logic is this.
On the O-line, whatever decent-enough talent might be left in free agency to serve as the default if the 21 year-old fails to win confidence… they're probably more keen on finding a team that didn't just draft a FRP at their position. So, assuming there was anyone there you'd like, you're probably going to have to overpay to compensate for the fact that they probably have better options in terms of playing time.
In the WR corps, it's not very different, except instead of a FRP, you've got a guy who just signed a significant contract who is anticipated to be back at some point. And given the enthusiasm for the 3rd round guy who they almost took in the 2nd round, there likely are better options for a credible free agent receiver (… and again, like O-line, not that there's that many left to pick from anyhow).
Finally, among the LBs, it's not even clear that there would be any real chance to start, so LBs with that ambition (ie, who are actually potential impact players there) probably will have some allergy to signing up with Big D.
So, I'm one of those persuaded now that you watch the trade block for some decent opportunities instead of free agency. Some names are beginning to be floated among team bloggers/reporters as vulnerable as a consequence of their teams' higher draft picks.
Some early candidates on the O-line specifically include Dalton Risner (OG, Broncos) and Denzelle Good (OG/OT Raiders).