CGI trailers

okay so i got a simple question wich might not be so simple, first of all why cant games be like their CGI trailers? i mean graphically? dont we have the machine power now on PC's to run games that look exactly like CGI trailers? or am i delusional ? would it take nearly a decade to create a game with CGI type graphics? or whats the limitating factor here? companies dont wanna spend so much money on real cool and nice looking games? help me clear some doubts lol because some of the recent games, all genres have awesome CGI trailers wich in my opinion would make the games that much more enjoyable but what do i know right?! lol