Howe has been here 3 and a half seasons. Look at what the 3 managers you mentioned spent in their short time at Man Utd. Then allow for transfer inflation. Howe has didn't the 8th most in the PL since he's been here. Your 3 were in the top 3 spenders when at Man Utd
https://www.transfermarkt.com/manchester-united/alletransfers/verein/985/1000/