They love pollen and nectar, but are not good pollinators. Many plants have protective devices, like closed throats or sticky hairs, that are designed to keep these robbers out. Hiding nectar in tubes and closed blossoms has many advantages. If it is hard to get at, only good pollinators can find it. Also it is protected from rain and wind and heat and cold.
Some plants have hairs on their stems that keep robbers from even reaching the flower cluster.
Some plants excude sweet liquid on their stem when they are flowering, so as to sidetrack the robbers.