Bees and biodiversity vd greenhouses

Why is pollination and the role of insects such as bees important, and preferable to artificial pollination, fertilisation, and just growing everything in greenhouses?

Before anyone asks, this is not for a course, I am asking for my own interests and purposes. Nor is it for the essays I used to write online, I have struggled for so many years to understand these basic questions now that I have come to the conclusion that I am lacking in knowledge on ecology, plant life and the whys and wherefores of pollination and its role. For instance I have asked on this site before, what good am I doing by attracting bees t my garden, are these the same bees that will fertilise crops in farmers' fields, and in fact if so surely I am luring them away from such places by attracting them to mine. The general consensus was that we do no good to the planet by attracting bees and that there s a surplus of plants, not a shortage. A colleague was saying recently that we could lose all bees and just slap up greenhouses everywhere and use hand pollination for crops. I wanted to plead the opposite case but could not find the right words to do so. I'm assuming it would cost a lot in terms of equipment, time and wages.
As for greenhouses, I posed this question to someone and they just said, "What about the greenhouse effect?" but would not elaborate. It seems the Eden Project has been successful and it is run along the lines of turning the whole area into a glorified greenhouse.
Simple answers only please. I am a beginner in these matters and would just like to know more about ecology and how we can help it, though I'd appreciate a bit of detail rather than just something along the lines of "We can never do better than Mother Nature," or links to books - I work in libraries, believe me I have read all the famous books on these subjects and I do not understand them.