At least implicitly, their estimation is that accepting your money would damage their ability to exclusively license content 28 times in the EU, and that eroding that system would do more harm to the bottom line than the extra money they might make.
In practice, the vast majority of content is of course not successfully licensed 28 times, so they end up turning away/not even aiming for the majority of their potential customers for the majority of their works.
Whether the current model is really best for rightholders' (maybe just publishers') bottom lines is up for debate -- in their calculation, they are probably biased towards prolonging the established business models that come from the times of terrestrial TV broadcasting/physical media sales.
What is clear, from an EU point of view, is that the geoblocking that is required to enforce this model artificially limits the audience of a lot of European cultural products, forces people to seek out copyright-infringing channels, reinforces digital borders between EU member states, harms lingustic minorities etc -- and is just in fundamental conflict with both the idea of the common EU market and the internet as a global medium.
(Disclosure: I made the campaign site.)