I'm sure people, especially first-time Morgan purchasers, do look for low-mileage cars, I think it's possibly because they might not know much about Morgans or cars generally, and so (not entirely unreasonably) assume that a car that's not been used much has had less time to wear things out.
Once they learn more about cars, they realise that this is a false logic, and do the sensible thing which is to assess on overall condition as others have said.
Fortunately for the Morgan trade, there's a steady stream of new purchasers who are in the first category, and it's good for purchasers in the second category as they are able to buy better cars for less money.
My car is 32 years old, has done 120k miles, and is probably worth around £20-25k to the right buyer - which would have to be someone who values performance over appearance

. Would I have more fun or enjoy it more if it was worth twice as much? Would I swap it for a new Plus 4? Definitely not.