two primary reasons: hubris and greed.
The hubris to think we know which varietals are the best and will continue to be the best. We may go all in on one species or variant and then turns out an unknown bacteria we have previously no clue about wipes out all of them. You never know, you need variety. Bacteria outnumber us all.
number two: greed. You worried about tech being consolidated into the big 5? how about this scientific research? you want our food, something we actually depend on, to be consolidated into 2-3 chemical companies? I don't.
Smaller reasons include: the power of being able to still survive on pure nature's means, and the freedom to do so. We don't realize it, but these things we do out here in the more advanced nations greatly impact the developing world, where a large portion of the world's population exists.