There's a few reasons.
1: Many people buy games that look good. And they look good with good graphics and high-intensity "cinematic" scenes. These scenes are much easier to do when they're scripted, and the player has limited (if any) control.
2: Console hardware is very limited. To get the good graphics for those cinematic scenes, developers have to make compromises somewhere. And framerate is always the first thing to go.
3: Developers then use the excuse of "30fps is more cinematic" and try to convince us that 30fps was the right choice all along. This is a blatant lie. If any dev that says something like that, what they actually mean to say is "We want our game to look really pretty, so we nuked the framerate and hindered the game's playability in favor of some extra eye-candy"
4: Reviewers are also to blame here. If a game has good graphics, it always receive praise for it. Good graphics is guaranteed to earn a game at least some points in a review score. However, they'll rarely (if ever) praise a game for a good framerate, and framerate is never really brought up unless it suffers from frequent sub-30fps dips.
"Never argue with stupid people. They will drag you down to their level and then beat you with experience."
-Samuel Clemens
