That's a good question and I'm not 100% certain you should but there's merit and here's why...
1. Just in career value alone a good to very good OT is going to be around longer than a good to very good RB. Do you want to have to drop a 1st round pick on another RB every 4-5 years?
2. The possibility of injury is so much greater now for a back than it is an OT. Yes offensive tackles do get injured but take a look at the list of backs that missed time last year due to injury... And you can start with Murray– Bell, Forsett, Charles, McCoy, Lacy, Foster, Lynch, Forte, Ellington, Hyde, Rawls, Yeldon, Spiller, Bush... And there are others. The present concussion protocol is going to keep backs from playing 16 games. OT's? Not so much.
3. Even if you drafted a RB #4 are you going to get full value out of him? History is unkind to backs that get 300+ carries for more than a couple seasons. If you're going to drop a #4 pick on a back to give him 250 carries is that good use of resources when you can get a 1,000 snaps out of an OT?
4. Replacement value... There should be no argument that replacing a good to very-good OT is more difficult than replacing a good to very-good running back.
5. Current value in relation to salary. A rookie running back coming in is going to get $24 mil in salary over the next 4 years, which already makes him one of the highest paid at the position. A rookie OT coming in is going to be getting the same money, which makes him a much better value in relation to the paychecks received by good OT's throughout the league.
Again, not saying OT is the way to go, but there's merit in at least considering it.