Because we were also desperate for CB help.
We've spent a lot of resources under Garrett addressing the OL. Both high picks and the churning with did with mid and late -round (Nagy, Arkin), college free agents (Leary, Costa, Kowolski and veteran free agents and trades (Mack, Livings, Parnell, Cook). That's on top of the three recent first round picks.
You have to keep the context in mind re: how few resources we've had in terms of draft picks and cap space since that 2010 season when everything fell apart. We needed to retool both the secondary and the OL on the fly and tried to get by developing interior OL talent early on the cheap. And even that was challenging when Nagy, Costa, Kowalski, Livings, and Mack all went down with significant injuries while we were juggling and changing protection schemes.
It's fine to complain about decision the team's made (not that you're doing that here), but you still do have to keep those decisions in a proper context. It's not like we had CBs stacked like cord wood and were out drafting more 'just in case.'