How is infect a fair mechanic?
Infect isn't just "dealing 10 damage is enough to win". It's "dealing 10 damage with creatures with Infect is enough to win," which means that you have to go all-in on Infect to do well with it. This means that Infect decks are very creature heavy and are very vulnerable to board wipes; a single Terminus can set you back several turns. 

Infect creatures are also generally easier to remove than other creatures. A total of 4 Infect creatures have abilities that make them more difficult to remove: Blightsteel Colossus, Skithiryx, the Blight Dragon, Tel-Jilad Fallen, and Toxic Nim. Every Infect creature can be removed with Path to Exile, and most can be killed with Lightning Bolt.

Plus, besides Blighted Agent, Infect creatures can be blocked like any other. My 2/2 Infect for 3, like Contagious Nim, still trades with your Grizzly Bears. And that extra mana can make the difference. By turn 4 or 5, the player not playing Infect should have creatures that are bigger than the player playing Infect. And attacking into a group of larger creatures still ends up with your creatures dead, even if they did have Infect.

Overall, cheap Infect creatures with cheap pump spells can get close to victory shockingly fast. But they can also be stopped by cheap removal and countermagic.



Blightsteel Colossus is a special case: it costs so much that Infect decks don't play it; instead it's mostly played as another win condition in decks that can cheat creatures onto the battlefield, and those also play other ridiculously powerful creatures like Griselbrand, Iona, Shield of Emeria, and Emrakul, the Aeons Torn.