I discovered this for myself some years ago when I wrote my first book. It was an introductory guide to antique ceramics of a particular sort. I had read the books available, talked with collectors and dealers, and photographed their examples for inclusion in the book. It was a lot of work, but it was not an exhaustive study. However, once the book was published, some people began calling me an "expert" in that subject. I refused the title and continued to learn more, which was used in later books on the same subject. I still do not consider myself an expert, and I've covered a lot of ground in the antiques, collectibles, and art fields. I highly recommend refusing that title when someone tries to bestow it upon you. I also recommend avoiding the temptation to take on that title yourself.
This explains why so many experts when interviewed are entrenched in their positions and will consider no other point of view. That title has done bad things for their ego. In my humble opinion (and that's all it is), those who attempt to bestow upon you the title expert are those who do not want to do any research themselves. Those individuals who accept the title are often people who no longer wish to do any further research and are declaring themselves done. Those are both gross generalizations and do not apply in all cases. I'm certain some wear the mantle of expert far better than others, remaining humble and inquisitive throughout their long and productive careers. We should all be more like them.
Disclaimer: This short post in no way elevates the idea that we should all be ignorant. Their is plenty of people promoting that idea in what they say and do. Many politicians promote ignorance when they use the dodge "I'm not a scientist ..." right before saying something profoundly ignorant about a field they do not understand, will never be mistaken for an expert in, and have no desire to learn about as it would challenge their preconceived notions. Instead, learn all you can about what fascinates you but stay humble. True wisdom is discovering how much more there is to learn, no matter how much you have already discovered.