because GTA was arguably the one of the first games to truly let the player do whatever they wanted and the series as a whole has set a high standard to follow
lets face it though Saint Row is a GTA clone totally-it was even made by one of the guys who developed GTA
whenever a game sets a high standard, in gameplay or innovation, it will inevitiably draw comparisons with other games that follow
shooters are often compared to halo
stealth games compared to MGS or Splinter Cell
you could go on
even the new FIFA game is being compared to past Pro Evos
imitation is the best form of flattery