As stated, 33's are really just the size most people go with because they fit with very mild modifications on most Toyota 4x4 trucks and SUV's here in the US. Also, generally speaking, the US toyota drivetrain's tend to stay pretty reliable with a 33" tire (though not always). For overlanding, reliability without excessive modification seems to be most people's preferred method.
Personally, I think a 40" tire is optimal for wheeling/crawling, but a 32-33 is a great size for highway use with offroad capability in mind. I MUCH preferred the way my vehicles drove, and handled, on 33's compared to when they were on larger sizes. However, the capability of the vehicle increased quite a bit with the increased ground clearance. So, as with everything, it is a trade-off.
Additionally, a 33" tire, while still expensive, is still a reasonable priced tire to be wearing out every few years (depending on how much you drive). Much over 250 dollars a tire gets to be a real strain on the wallet for a daily driver. At least to me.