Immigration is some of it, but a larger part is what I call the big industrial size farms. They produce food by the truckload. I have literally driven a rig into a lettuce field, a tomato field, a potato field, etc., and picked up produce. Most of those places are harvested by machine now. I would be one of hundreds of Tractor Trailers in one field.
Mom and Pop farms, and Organic Farms have to charge more based on volume, use of land, seasonal issues, etc, but on the other hand the produce is usually better.
As with most things, you get what you pay for.
Yes, definitely. They pick stuff early and gas it ripe, they breed plants for their ease of transportation. But those vegetables taste terrible.