Because the coaches thought Costa would be OK last year and he wasn't. They cut Gurode without having an adequate replacement for him which is downright stupidity. There were viable FAs available both last year and this year but they didn't go after any of them opting instead to go with Costa and Kowalski, two completely unproven players.
They obviously see something in Costa that appeals to them but we have yet to see that translated to the field of play on a consistent basis.
The assumption is that the coaching staff knows what they are doing (that's what they get paid for)but sometimes they don't, sometimes, believe it or not, the fans can see things that the coaches can't. It's rare but it's true. Coaches can get stupid about certain players or positions and get locked into a set pattern of thinking that blinds them to reality. I don't know why it happens but it does. We saw it with Tom Landry in a number of cases and we see it in every coach over the years.
Sometimes they get so set on having a guy that meets a particular set of criteria (size, speed, quickness, long arms, big hands, fat keaster, red hair, whatever) that they are blinded to the player's limitations on the field of play. Landry liked players at a certain size for each position and passed on guys that were better players because they didn't fit his ideal for that position. At RB, for instance, he finally broke from the mold when we drafted Calvin Hill and we had our first 1000-yard runner.
I am sure Garrett has his own strange criteria for players that works most of the time but not always, and unless he is willing to let go of it and see things clearly, without the filters of his criteria, we will continue to see the poor judgement like with Costa last season. Hopefully the addition of Callahan will help alleviate that condition and Costa will improve dramatically this year.

ray: