Camper64 is absolutely correct. It has everything to do with building code. If I had built the shed on a foundation I would need to buy a permit and get inspections. It would be a permanent structure. The construction of the shed would never pass code not that it's bad. I would also need to spend more money than I have on a shed. There are people on here that have much more money than I. The area we live in requires you to dig 3' down to place a footing and then a foundation wall. I would have to hire an excavator to dig for me. I'm to old the do it by hand. Then pour a slab and or build on top of the foundation. An 8' span would require 2X8 floor joists.
"Those 2 little 2x4s between the floor joists and pad" are pressure treated lumber. The same sill plates you put between the concrete foundation and the floor system of your house. If they rot away I can just jack the shed up and place new ones. The concrete pad has a slope so any water will drain off, plus the wind here blows almost constantly drying anything under there out. It's a dry climate here we get less then 12" of rain a year.
If you drive around your area neighbourhood you will see construction where the floor cantilevers past the bearing wall and does just fine.
I am a professional I worked as a carpenter for thirty years. I have built everything from dog houses for rich people to mansions for people that have way more money than I do.
Finally if I ever move I can take it with me it is only a shed after all.