Because most pro players play most of their games in custom games, and I would not be surprised at all if they had access to a dedicated server for pro matches that doesn’t suffer from the sane issues as the standard servers that are overloaded.
I’ve said this before, but it isn’t the server code that’s the problem it’s the number of virtual servers per box that Riot has the data centers running. The issues I’ve seen were not a problem worth mentioning during the beta, you’d occasionally get a shot that didn’t hit or registered wrong (impossible to avoid completely in any online shooter).
However, ever since the official launch I see it happen more than once every single game. I’ve been playing FPS games for over 20 years online and I’ve played at a highly competitive level in leagues on a number of teams. I.E. I know when my shots hit and when I missed, even if I discounted fully half the times I see it happen to me or one of my teammates as I just “saw wrong” it still happens far too often to be ignored.
I really wish Riot would follow Valves model and allow private/community servers. Aside from enabling actual third party amateur leagues (which helps produce talent) it would also enable community maps and mods, which has been a huge boon for the CS community over the years. There’s a reason no other FPS has ever come close to matching the popularity of CS on PC.