This isn’t necessarily a BIAB problem… although it often is because people don’t bother to sparge when doing BIAB. But I do.
If you sparge, I find that regardless of whether you BIAB or batch sparge, gravity points plus efficiency is almost a constant. For example, if you normally brew 1.060 beers and get 75% efficiency, your constant will be 60 + 75 = 135. Using this value of 135, you can back calculate on any other recipe of X specific gravity to determine the anticipated efficiency. So for the same brewer with a constant of 135, if they want to brew something of original gravity of 1.080, they’ll know that their efficiency will be closer to 135 - 80 = 55%. This is a little conservative and they’ll probably actually get closer to 60%, but my point is that there is a very real balance between the two things.
If the same brewer is getting 60 + 75 = 135 but doing BIAB without sparge, the effects might be more dramatic, since a sparge would help. Maybe they really will only get 55% for that 1.080 beer. So even a simple dunk of the grain bag in hot water for a couple of minutes might be all that’s needed to bring efficiency up another 5 to 10 percentage points.
That’s why I sparge, even though I BIAB about 95% of the time. The only time I don’t sparge is when brewing something less than 1.045, where it really just doesn’t matter because my efficiency hits 90% or more without even trying.
Crush hard, and sparge well, and you will be very happy.