Tetris gets faster because that way with every level the challenge rises, so sooner or later every player gets to the point where the challenge is equal to his skill. If you can consistently get to level 10 in Tetris, you are a better Tetris player than somebody who can only get to level 5. Your skill goes up while playing, because you learn, but the challenge goes up quicker, so every single game of Tetris ends with you losing, and the level at which you lose is a measure of your skill. Then you start over, and because your skill goes up, maybe in the next game you get one level further. If you would draw a graph plotting the maximum challenge you can beat over time, you'd see a constantly rising curve, but which is getting flatter, until it levels out at your maximum potential.
Now imagine MMORPGs would work like that. You start with a level 1 character fighting level 1 mobs, and you would *NOT* get stronger by gaining levels and equipment. If your character would always remain at exactly the same strength, and only your skill and random factors like critical hits would determine your success, then the maximum level of monster you were able to kill would say something about your playing skill. Some players would be able to kill level 5 monsters with their level 1 characters, others would maybe be able to kill level 10 monsters. But obviously you'd never see 25 level 1 characters raiding Ulduar.