Why didn't the Bengals add a proven RB in the offseason

[*ochocincos*]

Why didn't they?

They'd only be able to speak for certain, but it appears they felt Mixon was going to be the lead RB and they just needed depth guys.
They drafted a guy they liked and felt had strong potential down the line in Chase Brown. Brown has speed, he showed he could be a bellcow if needed/wanted, and he demonstrated a strong willingness to pass protect. They thought Trayveon Williams had enough talent and experience in the offense after 4 years that he was worth retaining. Chris Evans is showing like he might become a third-down guy that can not only catch (we knew that) but also (now) be able to pass protect.

In regard to losing Perine, Perine offered good value as a 3rd down guy, but he definitely wasn't fast. Taylor's crew clearly values speed, as evident by all the draft picks they've made.

With all that said, there are still some RBs out there to pick up if it's getting into the season and it's appearing that none of these backup RBs are getting it done.
In terms of rushing, I see both RBs rush for very little YPC against ATL. I wasn't able to watch the game (out of market, not on Sunday Ticket), but when multiple guys perform about the same, I tend to lean toward the OL not getting much push at the LOS and/or getting holes opened.
In the first preseason game against the Packers, Brown rushed for just 1.5 YPC. Evans ran for 2.67 YPC outside of his 33-yard gain, which had boosted his YPC to 7.0 that game.
In these games, the Bengals haven't had their starting OL in, and these backups just are not strong (run) blockers, for the most part.

I am optimistic that, at least from a rushing perspective, we'll see these backup RBs perform better once the actual real OL is out there.