For every ordination, a bishop presides over the service, but the bishop isn’t the only one who needs to be present in order for there to be an ordination.
For Deacon and Priest Ordinations, there needs to be a bishop and at least two presbyters (another word for priests). In the Priest Ordination, the other priests come around when the bishop calls upon the Holy Spirit as a reminder that the ordinand is being ordained as a fellow presbyter with them.
When a Bishop is ordained, there needs to be 3 bishops present, one being the Presiding Bishop or a bishop appointed by the Presiding Bishop. This is a reminder that the bishop is not subservient to any other bishop, but is called to mutual leadership with their fellow bishops over the church. Having the Presiding Bishop, or a representative, serve as the presider of the service is a reminder that the whole Episcopal Church takes part in this ordination. To make this sense stronger, it is also required that there are representatives from the diaconate, presbytery, and laity at the service so that all orders of ministry in there.
With all these services, we are reminded by having multiple people present that ordination is not an act of a bishop, but rather an act over which a bishop presides. At the end, it is really just the action of the church as a whole.