Stamp collectors for exploration are quite a bit different from the kinds of collectors that tend to go to stamp dealers (I am one of those collectors).
You end up focussing on extremely narrow areas of the hobby to the point where you are hunting down which library has a painting a stamp is copied from or trying to track down an envelope sent by a particular person from a particular city.
You can have 10 different variations of a stamp that look all the same except under a magnifying glass.
There are collectors who have hundreds of thousands of dollars in their collections and all of the same single stamp. Just 1000 different copies of it.
The premier component of my collection is an exhibit on the 1982 Canadian Philatelic Youth Issue, a single stamp series (about stamps).
So for a lot of collectors (including the ones willing to spend thousands a year) it’s less an accumulation hobby and more a specific research project.
Not excusing the behaviour of the dealers, but meeting someone who collected so widely could be unusual for them.