we always bought a pallet or so of tomato stakes (bout $.11 per stake) and some baling twine.
throw a staple gun in the mix and have fun.

growing on the ground is only possible if you are REALLY careful picking, and wheat straw mulch helps keep the fruit from rotting when in contact with the soil... altogether not bad for growing, but labor intensive for picking.

Depends o the market. Many that ship to processing plants and have mechicanical harvestors don't use anything. The people sending to retail markets usually stake, cage or use a mulch to keep the fruit clean. Some growers use mixed methods as some varieties work better with different systems. Keep in mind that many of the commercial growers pick the tomatoes when they are still green. That green tomato is firm and free of the ripening blemishes. It will withstand the rigors of washing, packing and shipping. Do you want that tomato on your sandwich?

When growing large quantities of tomatoes, I've used mulch and I've staked. Even with mulches one has to be very careful of rots. Cages have never worked for me, as I use the indeterminates and they exceed the cage heights, and even the heavy bearing determinates can take a cage over. Stakes are time intensive, but seem to be worth it. I've also run temporary fencing.

Like mentioned for very large scale production the tomatoes are hard/green/and boxy....bred for mechanicals. Market gardeners on a smaller scale ask more for their product, and should expect to invest more into it.

I plant fresh market tomatoes at 4000 plants per acre, thats a lot of cages to handle for just one acre!. We use treated 56" stakes with the Florida weave system that uses 2000 stakes per acre or one stake for every two plants.
This is the standard staking method of the industry... Bob.

What are these stakes treated with?

nothing here...
either get pine for one year, or cypress for 2-3