So that you don't kill somebody.
Seriously, garage doors can and do kill or seriously injure some people every year, although many fewer than they used to precisely because of safety standards like this one.
There are all kinds of reasons why someone might be in danger from the door without activating the door's own beam detection.
Both dogs and little kids tend to hesitate right at the threshold where the door comes down, probably because of the change in lighting. Either one could be on the ground at a level where they won't block the beam and either one might not be aware that they had to move out of the way once the door began moving.
There's also a very common issue, which has actually happened to a couple of people on some of the other home automation forums, where someone parks the car so that the trunk is in the pathway of the door but the beam has not actually been broken. This might be because they are unpacking packages from the back of the car or loading up suitcases for a trip.
The garage door may come down and hit the open trunk, forcing it down even as the garage door itself retracts because of the impact.
Other issues as were already mentioned include someone staying at the house who is doing painting or yardwork and wants the garage door to stay open for awhile.
Anyway, anything which automates the garage door without requiring a visual check by a human introduces these kinds of issues, which is why many people, particularly those with young kids in the neighborhood or escape artist pets, will tend to not automate closing of a garage door when there was no human verification that the path was clear. And why the big garage door companies, who know more about these issues than anybody else, generally don't automate door closing without human verification.