Well, basically, they made a bunch of swords. It's a testament to Rome's administrative and economic power at the time that they were able to do that. They didn't always do it and the number of equipment produced as well as its quality fluctuated throughout the years but basically they were in a position few other empires, kingdoms and states were where they could produce so much equipment for their army. They still had soldiers who were equipped less well than the rest, most soldiers did not have armor, and for most of Rome's history, soldiers had to pay for their gear out of their own salary rather than being issued from the state, though that did happen for a period.

That being said despite most soldiers having a sword the spear always was the main infantry weapon for pretty much all armies, or if not a spear, some sort of pike or other polearm, pretty much all the way until firearms were widely adopted.