Why do people in the UK care about America politics?

Because of the danger of the stupidity and dumb ideas taking hold over there.  American taxpayer dollars are wasted like never before on criminal aliens, PCBS like studies to defund the police.  The transgender nonsense where an entire population has to be inconvenienced because  38 guys decided they are girls and want to run on the woman's track team.  Then there is the public funding for guys who join the military and want to be girls at taxpayer expense.   It's horrifying living here watching a once great nation so side tracked in nonsense that we can elect a noted idiot who at age 78 has the brain power of an 88 year old dementia victim.   

Probably because American politics is like watching the circus, it's full of clowns.

Their politics are boring 

Don't know, just like why do some people in the U.S. care about British royalty,,

Because they like to see stories when citizens use a gun and shoot criminals. Because criminals over there get away with crimes because there's no one to stop them.