Horses for courses. You could say that about anything really.
Some people service their own cars, most use garages. Some people do their own DIY, other people use tradesmen. Lots of people engage gardeners and cleaners, but "could do it themselves".
Not everyone has the ability, time nor inclination to do the research they'd need to do to get the same level of financial advice they'd get from an IFA.
I'm an accountant. Lots of my clients "could" do it themselves, but they choose not to and pay me instead. Some sway in and out between years from using me or doing it themselves, depending on how much time they have spare and whether there are any complexities in a particular year, such as capital gains tax, or if they want an IHT update review.
What I'd say is that level of "wealth" isn't really the determinant factor. Lots of very wealthy people can manage their own finances especially those with time on their hands after retirement or selling a business, and lots of relatively "poor" people (compared to the very rich) use IFAs especially if they are young, have families, working full time, etc., and simply don't have the time or headspace to start doing hours of research.