Why every second continued game is more fresh ?

I mean Infamous part one is somehow not significant and gray colored and part two is opposite of it. Also Assassin's Creed part one is also! same as infamous with just bit less juice than AC 2 and 3 had the missions are LOT more extensive than part 1 have and like everyone mentioned here its more COLORFUL (I got this from own experience, I just play both part 1 and 2 today and I really love second part). I guess Prototype 2 will be also more action and shiny. They could delete these parts and make one all-configured game :) (lol impossible). I dont have nothing to it but I cannot just overlook this one ! What do you think guys ?