Which teams are you referring to? They haven’t played the same low level teams have they? The only decent team that Romero has kept a clean sheet against this season was Wolves. De Gea kept a clean sheet against them too.
Romero hasn’t been a number 1 anywhere for about 8 years. Why do you think that is?
Argentina Number 1 no? (top 10 most capped is freaky).
Can't comment on club levels, don't know the contexts of all the different stories at each club. If I have to speculate, it's due to his personality... easy for managers to drop him to No.2 in favor of other keeper who will kick a fuse if dropped and make problems being ego demanding to play.
Using only one season (the most recent one) as a sample is limited. Using all the past seasons since Romero joined 15/16 (so both him and De Gea are playing with the same teammates) will yield better data. Additionally the EL teams they faced.
Just compare the (1) PL teams they faced (De Gea mostly in PL but some in cups and Romero mostly in cups). Mid table decent teams, lower mid-table teams and low level teams almost always don't rest their players in cup games so full strength. Usually-but-not-always the top teams will play their 2nd fiddles. And also compare (2) the Europa League teams they faced.
Romero's record overall seems to be better against those type/levels of teams.
Interestingly, Romero's records vs the PL top teams (albeit very few games in comparison) is surprisingly good.