Since 2005, Wenger has consistently had the 3rd+ spending power in the league, and had rivals (like Chelsea and City) who would lure their players and targets. For him to come close to winning the title a couple times, consistently making the top 4 until his very last season, and going deep in CL until the late noughties is a great achievement.
He is no miracle worker like Klopp, but United fans shitting on him is weird, when the club can't be consistently reach top four after being the 2nd highest spender in the league (and significantly more than the club below them)