In recent years, games have been turning from a linear, story based campaign towards a more open world approach with the story happening to simply wait around while the protagonist goes and screws around for about a month in-game. Because of this, it can really be detrimental to the urgency that most plots want to give a character. A good example of this would be the Elder Scrolls series, especially Oblivion. We have the protagonist, who out of sheer luck has escaped from jail because his cell has the secret royal escape rout entrance in it and the emperor needs to use it to escape. When he dies, he gives you the royal amulet and instructs you to take it to the leader of the blades. At this point, the entire world opens up and you can do whatever you want and as long as you don't take the amulet to Jaufree, the plot doesn't move on. The oblivion gates never open (except the one in Kvatch) and no one dies unless you are present.
So my question is this: why don't more open world games have some form of time limit? A good example of an open world with a time limit is the dead rising games. You are given 72 hours in-game to do whatever the hell you want. If you want to do the questline, you have to do it at appropriate times and if you are late for even one quest, you fail it all and are left to just do whatever you want for the remaining time.
Now I know that a major part of most open world games are the exploration aspect, and I also Know that without that I probably wouldn't like them as much. What I'm asking is this: why can't there be some form of invisible time limit? An example of this would be you are given a quest to go save a person from being assassinated in another town. You are told that if you don't hurry, you will fail. If you decide to go screw around for a bit, the assassination takes place, but you don't know about it until you arrive at the location. Also, on the subject of urgency, why isn't there more "in 3 month's time so and so will invade. we have that long to prepare" types of quests? it would allow you to do whatever you want and depending on what/how much you do depends of how the final battle plays out.
tl;dr Why don't more open world games have more time limits?
edit: tl;dr#2 Why don't more games have plots that fit open worlds without time limits?