Because these are the players the Cowboys have drafted in this century when they've had a Top-12 pick:
2002 - #8 Roy Williams (a few GREAT seasons at safety when he was playing with Woodson)
2003 - #5 Terence Newman (very good CB. Elite during the first 4 years of his career)
2005 - #11 DeMarcus Ware (future HOF)
2011 - #9 Tyron Smith (elite LT/maybe future HOF)
2012 - #6 Morris Claiborne (bust) - via trading up
2016 - #4 Ezekiel Elliott (one of the best RB of his era. Also could have gone with Jalen Ramsey, elite CB)
That's a pretty good track record overall. If we can use this pick to get another future All-Pro caliber player on D, that's how you begin to rebuild it.
Key point is that the likelihood is even if we lose out chances are we won't draft higher than 4th and probably no worse than about 6th even without tanking. SO we are gonna get a chance at getting a really good player. As for tanking, what is the recommendation. Are you people suggesting to actually try to lose? In reality, we are already playing a team full of backups. On offense we have LT, RG, RT, TE and QB out. On defense, our secondary is guys we are pulling off the street. Our DT's are bottom of the roster level. I mean we could go ahead and sit Dalton, Zeke, Cooper, Gallup and Lamb. Take out Tank and Gregory and LVE....but I doubt it would make that much of a difference. Of course that means you are sacrificing those guys chances of making performance bonuses. You are not giving the young guys a chance to develop with better players around them..you are kind of just throwing them to the wolves. Not sure how tanking really is a positive in any way.
As for the draft....we have huge needs on OL, DT, LB, CB and S. We shouldn't have any difficulty picking up the best player at one of these positions. Now let's look at some of the later picks and see how they have worked out.
2020 - #17 - Lamb, looks like a special player
2018 - #19 - Vander Esch, sometimes looks special, but injuries make this a ?
2015 - #27 - Byron Jones, Pro Bowl level corner, we let him walk. Should we have paid him instead of Jaylon, Zeke or even Cooper?
2014 - #14 - Zack Martin, perennial Pro Bowler, likely Hall of Fame candidate
2013 - #31 - Travis Frederick, perennial Pro Bowler was the best C in football before GBS
2010 - #24 - Dez Bryant, was among the best WRs in football for a 5-6 year span
So...you still can get really good players even if drafting further down in the draft. We have done it in a lot of our most recent drafts.