The sites with millions of hits not A/B testing might be helped by an A/B test but more likely will just chase ghosts and give it up.

Or they could just hire a good UI/UX and conversion expert and get the insights for free.

Take agriculture - you are running a large farm which is hundreds to thousands of acres. Do you run experiments with soil, seeds, fertilizer, crop rotation schemes to increase yield? No, that would be silly, take forever and make you poorer and perhaps no wiser. Instead, you hire a "expert" who has done it before and knows the "best practices".