While all the reasons people have posted about Hollywood being rich to afford lot of writers and British shows being largely one-writer show seem right and logical, the one I read somewhere long time back (and like the most):

“Americans like the happy ending hence Michael Scott has to eventually come across as a good guy but British are more realistic i.e. life is f**ked up and it’s going to remain the same” - so they don’t stretch things to the point that it becomes cringey and leave it at its creative peak.

(Can’t find link to the article)