Buried deep in the OCA website is mention of a claim to being the US National church. Their presence in lands now part of the US starts before the US, as part of the Russian Orthodox Church.
The problem is that there was little overlap for many years… now that there is, the others are unwilling to conceed their churches and hierarchs into the OCA synod, for a variety of reasons.
Further, the OCA are autocephalous. Their metropolitan is the primatial bishop. Given time, I would expect, eventually, for their metropolitan to be acknowledged a patriarch, but not while there are 5 or more unaffiliated hierarchies.
Further, while OCA Autonomy is well accepted, OCA Autocephaly isn’t universally accepted, in part due to the canonical irregularities caused by 5+ orthodox churches operating in the US.