There's a reason a lot of those guys are in the portal. Most of them are because they have been processed.
Let's say Benhart entered the portal and we are all familiar with his time and struggles at Nebraska
If you were a fan of say......Washington St, and had no knowledge of Benhart's performance at Nebraska. You'd look at this recruiting profiles and see his size and his star rating and say "that guy's gotta be better than what we currently have"
My point is that linemen that can really contribute are not a dime a dozen. Everyone knows that it begins and ends up front. So if a linemen in the portal hasn't been picked up at this point....he's either REALLY good and can wait it out and the schools won't care or he's not a power 5 kid.