Because of the way party conventions and nominations work, with the way the US system is currently designed there would be essentially no way to get parties to elect their officials, tell everyone about the candidate, and then vote for a half term.

Don’t forget that the entire UK could fit into one US state and the entire population of the UK is smaller then the population of Texas and California alone.

Further, because of the sheer size of the United States, many people have to travel could need to travel upwards of 30-45 minutes one way to vote, so many people rely on mail in votes, which would be close to impossible to distribute and then collect in the time the snap election happens. It would just be easier to wait until the next election.

It is also expected that the vice president would carry on the plans of the original president since they are from the same party, so there aren’t any major changes in course expected.