I think that is mainly because Guard, Center, TE, & Safety are not seen as high priority positions like QB, DE, OT, CB, are. So the only time a team will select one in the first round is when they are truly standouts at their position.
QBs will go in the first even when they are not seen as "can't miss" players because a team NEEDS a QB. Same with some of the others. Look at the mock drafts and you will see a lot of QBs, DEs, CBs, & OTs in the first round but only one or two interior linemen and safeties.
It comes down to a perception of need rather than actual talent/ability. It is a rare draft when 3 interior linemen are selected in the first round but not rare at all to see more than 3 DEs taken early.
RBs, WRs, & LBs vary with each draft so it's harder to make a general statement about those positions. You could have a bunch of them go early in one draft and very few the next.
All that said, I think DeCastro should be our pick as he is a rare talent at OG, rather than Barron (whom many mocks have us taking) who is NOT a complete Safety IMO. We can look for DB help in the 2nd or 3rd rounds.
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