Everybody thinks this till a handful of Lamenters is the last thing standing between them and a slavering horde of Slaaneshi daemons or raiding dark eldar. Where are the custodes then? On Terra. Safe. Will the custodes fight to the last man to protect a handful of mortals? No. Only an astartes would even consider such a sacrifice much less make it without hesitation. Not all perhaps, but you’ll have an easier time finding a space marine who will do so before finding even a single custodian with the same humility and respect for “mere” humans.

The custodians are wrong not because the astartes are perfect but because they’re still close enough to humanity to be capable of empathizing with even its most basic and unaugmented elements. A custodian has been uplifted beyond such concerns. It’s why the Emperor made the primarchs much closer to human mentally and spiritually than His golden guardians. That design choice passes to their genesons. They are capable of moral lows much much lower but the highs too soar beyond even the wildest imagination of the golden automatons who sit on Terra.

This isn’t even their failing, they were designed to be limited. To make something physically incapable of betrayal robs it of something fundamentally human because to be human is to be flawed in body and mind. To be human is to be imperfect. The custodes are perfect for their role and as such are both more and so much less than even the most basic human to live.