This is a non-point. Circumcision is a command to the parent to perform or have performed on their sons.
If God hadn't instructed parents to circumcise their sons, then I would have no bases to ask why don't argue with God about circumcision. It's very simple, try to follow:
1) God commanded converts to be circumcised.
2) God had babies circumcised.
1) God commanded converts be baptized.
2) You say baby baptism is invalid because of #1.
See your problem?