Because they never evolve! 2 competent players cannot exceed each other as the games are often luck based waiting for flaws in AI to bag goals rather then player error.

The ball physics have improved, but at a very slow rate and the player controls/skills just continue to be limited. Im speaking more in terms of pro evo then fifa here, as fifa seems to have more realistic AI, and tackling traits, whereas pro evo is more fun but full of badly coded team formation AI.

Simply put, you just cannot dominate and be satisfied on a football game like you can on quake or ut, or whatever first person shooter you play, or even rts like warcraft or starcraft, as skill is far outweighed by luck.