Now they are used to allow vehicles to ford deep water, except I don't think a gas engine will run long when completely submerged. Really, they are there for show, not go.
It'll run submerged for a good long while if you've taken the time to set up the fording kit correctly. While not a truck, I waterproofed my Suzuki King Quad when I lived in Alaska and ran it through a lot of water crossings, and buried the engine in mud for 10-15 minutes at a time while I winched myself out. Engine kept running.
On truck snorkels I would point you at 1:40 of
this video. That Toyota runs for a while. (sorry about the music)
Yep, we had those on some HWMMV's. So you could drive through water over your head... rolleyes rolleyes rolleyes rolleyes
A HMMWV is rated for 30" water without a fording kit and 60" with the kit, so it's only over your head if you are pretty short. :)
https://www.youtube.com/watch?v=p9Jg5JR2FV0 I've done similar depths to this off a boat and up a beach, but you want to have nice firm sand, or better gravel.
Snorkels (well fording kits, of which snorkels are a piece) are very effective at allowing vehicles to operate with the engine and drivetrain submerged. I suspect that Americans on gravel roads aren't really the target audience for those TRD kits.
Snorkels also offer a modest Cold Air Induction/Ram Air effect which helps the engine.
As to Ben's comment, I see them all the time in the mid-east, but I think you are correct they haven't been factory option in the US. Land Rover offers one as a factory option on the Defender, but I don't know if they did here
ETA: Some quick googleing shows that you can get a Hi-Lux, A Landcruiser, a Landcruiser Prado, and a LC Pickup with a factory snorkel in Africa, the mid east, and most of Asia, so Toyota certainly feels they can sell enough. And people feel they are useful enough to buy.
Again, America probably isn't the target market, but someone in TRD was like "hell we have this already engineered for the Hi-Lux, lets tweak it a little for the Tacoma body and charge Americans a grand for it. Betcha someone buys it."