We have been down this road many times before. It's kind of tiresome, especially the implied negative commentary.
I find myself typing this response at least a few times a year.
1) No one wants to go on tour, slogging their guts out, playing two hours a night in a pressure situation, for months on end, playing a drum set they don't absolutely love and find uninspiring.
2) Most well known drummers have the opportunity to endorse most brands, so why choose something you aren't passionate about?
3) An endorsement is made up of many elements - global support (if you break that snare in South Korea with a show the next day), personal connections with the company, input on design and manufacture.
4) In the new century, the vast majority of endorsing drummers are on a percentage discount. They aren't given free gear, they get an artist discount. The top, top players may receive free gear. I don't know of anyone who is paid to endorse, although a company's willingness to pay fees for clinic tours and public appearances may add to a players income stream.
On the company side, yes it is about exposure.
35 years later people still message me about the Noble & Cooley drums I played on tour. I feel I played a large part in increasing N&C's exposure to the wider drumming community in the early 90's, but if it wasn't me it would have been another drummer. In fact N&C snares are more famous for having been the snare of choice for Phil Collins.