So the question becomes then why people believe so much this - "that those monopolies are too rich and powerful to be attacked by politicians" - to not act, while they, the same people, don't seem to have similar beliefs in other issues.

Usually because it's also in the interests of a powerful group who piggybacks off popular support for an issue for their own purposes.

Net neutrality was a good example of this - still good despite not being the most important issue du jour. Received attention because it's coincidentally in the interests of Google et al.