Not a professional, so correct me if I'm wrong, anyone out there, but...

1. Swords were not particularly expensive nor difficult to create. High end swords could be extremely expensive and difficult to create, but if you wanted every foot soldier to have a sword, that was pretty easy. But actually, asking how every Roman Legionnaire had a sword belies the more important and interesting question, which was how every Roman Legionnaire had a suit of armor. Compared to swords, armor, like the mail armor that legionnaires commonly wore, were extremely expensive and labor intensive to make.
2. The Roman Legionnaires were well equipped because of the unique way the Legions were raised. In ancient and medieval warfare, in almost all armies, the soldier brought his own gear. The quality of this gear depended on the soldier's personal wealth and the poorest of the poor usually had no stakes in the wars and so did not show up at the muster at all. Obviously, a slave (or slave-like poor rural farmer or poor urban worker) is still a slave whether he's slaving under one government or another. If your army gets wiped out, you generally have to surrender because your society no longer has enough men of wealth who can and will fight. On the other hand, the Roman Legionnaires were recruited from the poor and then armed at the expense of the wealthy patron who raised the legion. These patrons were massively wealthy and could buy a lot of excellent gear for their men and, further, had a great motivation to, because success in war could determine their advancement or even survival. If a Roman army got wiped out, another army could simply be raised from the wealth of another patron.
3. The Roman Empire was also extremely wealthy from its conquests and expansion, which made goods like swords and armor easier to obtain and in greater number.