Yeah, I find myself wondering about this, especially this season.
Seems like when the other team is on offense, their receivers often have a ton of space. This has been true throughout the year. I know the defense has been pretty good at stopping other teams from scoring, but our pass defense has looked pretty bad.
On the other side, why does it seem like our receivers are almost never open, or at least a step or two in front of the defender? As you mentioned, the spacing on offense has been an issue more often than not. The only consistent exceptions being the tight ends during the rose bowl years.