I think a lot of people have addressed the general lack of ethics wholesalers commonly employ to sellers so I will pass that over and address the issues investors have with wholesalers.
Many of the wholesalers in my market aren't adding any real value by generating off market deals through some campaign, they are instead locking up deals from the MLS, or FSBOs on Craigslist and then remarketing it themselves as a middle on the deal.
I have passed countless times on properties I have been in touch with the owner about directly because they have another buyer lined up who ends up being a wholesaler who has no intention to close. They will lock it up, misrepresent to newer investors the ARV and hopefully get a deal done, but it is more about throwing deals against the wall and seeing what sticks than anything.
I believe that a well put together and ethical wholesaler business can exist and that someone in that position would only need 3 clients to be extremely successful.
If someone in my market was able to put together a plan to provide me with consistent deals that had reasonable profit margins and understood enough about the process of flipping a home to do so, I would buy every single deal they could produce.
If you are daisy chaining deals together, you're in a whole new universe of non-contribution.