Believe them or don't. Or better yet, pursue it further independently. Learn why biologists think it is a compelling piece of evidence. Become learned in genetics yourself and pursue a career that explores genetic relationships and search for the elusive organism that shares no genetic history with any other organism on Earth. You'd be famous.
Just because many biologists think it is "compelling" (meaning that I'm "supposed" to believe it) evidence doesn't show that all organisms share a single common ancestor.
"organism that shares no genetic history with any other organism on Earth"
That's not what we are discussing.
There's a problem with the single common ancestor idea. Life is found all over the earth, from the ocean floors to very deep underground. How did life spread from a single location to all these locations? Isn't that just a bit iffy?
And what precludes life forming multiple times in a similar way?
So there's similarities in DNA. On what basis then is the 'conclusion' that there was a single common ancestor? What is the logical reasoning leading up to this conclusion? And don't say, go research it, no, you answer the question.