If Villa win the Europa League and finish fifth, then sixth place gets a Champions League spot. No such spot becomes available if Villa finish in the top four and win the Europa League.
So, let's say by some miracle we actually do win the Europa League. On the last day of the season Brentford already know that they are going to finish sixth, and travel to Anfield knowing that the only way Liverpool can finish above Villa is if they win. Brentford would be better off losing.