Steam came out before that. You missed the HL1 mod era.
I wasn't trying to imply they were released on literally the same day, of course steam was out before HL2, but HL2 is what drove its popularity because people were forced to install steam to get access to HL2.
And most people resented them for it. At that time, store purchases was still the common way to get at PC games.
Steam was the first “online required” for offline play I think. Others like EA and Ubisoft followed shortly after but didn’t have their own stores until many many years later.