It really wasn't. Early streaming (like 2012-ish, after cord-cutting was possible but before it was mainstream) had Netflix and Hulu as the two major services, and so anything that was on streaming was on one of those two. But lots and lots of stuff just wasn't on streaming at all.

If you subscribed to Netflix back then, you probably subscribed to both the streaming and the disc-mailing plan, because there were enough things you couldn't get on streaming, so getting the discs was important to fill those gaps.