I started to have same issue, I know most of my opponents not only in League but also in CUP or CL but this is not something new, those who are at high level can confirm you.
I also agree with you, depending on your style, this can be annoying, playing with same opponents can create a monotony which isn't exactly so good but on another side, some people like this...
You have a good point when you say is not random, yes is not random, the game is following some factors and one of the most important is quality. I prefer pure random but seems they don't, nothing much to do.
PS: Stop saying million of opponents because there was never million of them, not even at level 1. The game have a lot of groups and servers.