This idea is covered really well in Richard Garfield's book Characteristics of Games. The metaphor he draws here is what he refers to as the skill chain. Each

This idea is covered really well in Richard Garfield's book Characteristics of Games. The metaphor he draws here is what he refers to as the skill chain. Each link in the chain reflects the point at which the player on the lower link is unable to defeat the player on the higher link above some small percentage. The longer the chain the more skill is rewarded by the game. However this has a downside for an online game that isn't as popular as the ones you mention, you will be unable to find interesting games without having to wait for a long time for the correct opponent to appear. And it makes matching up well against your friends less likely. Randomness can be used as s tool by a skilled game designer to shorten the chain and allow for more player to compete with each other. It also gives the player an element outside their own weakness to blame when they lose and that can make the game more enjoyable. So it's something pro players tend to dislike but is typically accepted as the price of having diversity of skill in the player pool. (Poker vs chess for example)