Because the US is many times the size of the UK (in terms of physical size, population, and scope of it's government) and the US is made up of 50 governments with different rules and procedures. That's why it's called the United STATES of America.
The executive in the US is complete separate from the legislature; he is not elected by the congress, he is elected separately by methods determined by each state (they all happen to be popular vote today). And each state is a completely separate government with all the components of a Sovereign Government except currency and department with binding international powers. The time is given so every state can organize its Electoral Delegates and Congressional Representatives to go to Washington and any disputes over elections or a candidates eligibility can be worked out.
The parliamentary system of the UK is not on a rigid term schedule, they prepare for changes during the election process, have the elections, and divvy up the seats (or whatever you people do). As long as its within five years, you guys can do it anytime. The American process is on a rigid schedule elections every other first Tuesday in November and the following change of government comes on the following 20th of January; presidential election every fourth year.
I would cite regulation, but I know you will simply ignore it.
