This is one of the generalized reasons I work so hard to do my own work as much as I possibly can. If I can't do the work I can at least study on the problem so that I can get a sense for whether the expert is going down the right path or just pulling my leg. (refined my BS detection skill in the Navy :-)

Sometimes the experts don't know jack themselves and will make things up.
Sometimes what they know is wrong.
Sometimes the experts will break something else and not tell you.
Sometimes the experts get tired of explaining because some of their customers won't understand, won't believe them, won't listen anyway.

Sometimes the expert is dishonest and just wants your money. (like my local dealership who wanted to rebuild my rear differential and change a front ujoint when I TOLD THEM it was the front diff making noise, that I got lots of metal out of it at the last lube change, and the VIN was only a few hundred different from my other one which self-destructed at nearly the same mileage, and the diff model has a reputation for eating bearings. A pox on them! I went to another shop for the diagnosis and rebuild and have been treated fairly by them since. But I always check their work. Heck, I recheck my OWN work too.)