Because no manager anywhere has ever made a mistake signing a player he thought was good enough to move to a bigger club. Martinez definitely didn’t do that with any of his signings.
Well that’s a strawman..
Clearly managers make mistakes. But generally the manager who gets paid to manage football players and has actually managed the transfer target for several months is more likely to have an accurate understanding of the player than somebody who watched him play a few games on Match of the Day.
In my opinion.