My plumber needs to replace a 208 volt water heater and they are very expensive, $2,800 for a 40 gallon. I told him he can use a 240 volt unit but it will heat at 75% of what it would at 240. Why can't he buy a 240 unit and install 208V elements in it?

The best I can tell they are the same element, A 4500w becomes 3500w. That is how they show up in the catalog. Buy a 5500w 240v water heater.

Kinda makes you wonder why they make 208 units for 7 times the cost of a 240 unit.

Because they can. Maybe they have special elements that provide full watts but I didn't see anyone bragging about selling them.
There isn't a lot in a water heater. Typically it is just a tank with 2 elements and 2 thermostats. There shouldn't be anything special about the thermostats. I know they don't like salt water when my heater went under