This is kinda where I'm at with it in a way. The front office, in my opinion, must believe one of three things.
1. The Ravens aren't capable of developing young receivers. I would have believed this very much during the tenure of Jim Hostler and that would be supported by always bringing in these older, established veterans, but I just don't see it now. Engram is a really great receivers coach and we've seen great strides from players like Aiken and Perriman. I think if Engram were really given a bonafide star in the draft to work with (like Mike Williams), he could coach out any deficiencies and turn him into a superstar in the league.
2. The Ravens think Joe can do it all on his own and doesn't need a supporting cast around him. This one is just total bogus no matter who the quarterback is, except for one. I have only ever seen Andrew Luck do so much more with so much less and he was quite possibly the greatest draft prospect in NFL history, so we know that this isn't true.
3. The Ravens believe that no matter who they put out there, Joe is going to look exactly like he has for his entire career. Now, I know Joe hasn't had the greatest group of receivers in the world, but it seems that no matter who's out there, Joe is going to just make the same mistakes and the same throws. It doesn't matter if he has a Gronk or Julio; he's going to be who he is. He's gone through so many receivers, different offensive coordinators, and different philosophies and he's still playing the exact same. Sure, maybe if he had Gronk, Julio, Megatron, AJ Green, and Bell in their primes, he might be lighting up the stat sheet, but no quarterback has anything close to that.
I think the front office is resigned to the idea of giving Joe just enough to work with and not leaving him out to dry, but not hinging all their money on a receiver/tight end group that'd rival the best in the league because Joe is still going to be Joe no matter who is out there.