How does this make any sense? The customer has to pay the transportation costs of the plumber and electrician. If the customer only has to pay the transportation costs once, there is more money left for the plumber.
Presumably the water heater is not the only electrical thing in your house so if things are scheduled correctly (which is a really big if in the construction world) the electrician is coming out the same number of times regardless.
There is a specific order things need to be done. You need the fountain, then the walls, then the roof - only then can plumbers start. Because water runs downhill, the plumbers MUST be done next, as otherwise something else might be in the way of where a pipe needs to be. Once the plumbers are done, then the HVAC people come, and then after that the electricians come - it is no problem for them to put power to the water heater at this point - the water heater must already be installed. (or at least roughed in). This is how all houses are done, so there is never a problem unless all the electric is delayed.
In large buildings things are done different. Architects spend more time designing exactly where the pipes, HVAC, and wires go. Then the trades people work to spec, and they can thus work in any order so long as their parts go where they are told to. This is a lot more effort/cost though, so it isn't worth it for a small house but is critical for something large. A small house turns out to have plenty of space for everyone to design as they go, and thus this is cheaper. For a large building there are a few places where there is barely enough space for everything that must go through that spot so you better ensure upfront there is enough space.
Because most construction projects aren't "just" new plumbing... if you're operating beyond the "Yugoslavia way" aka renovating/constructing in pieces whenever you had scraped together some cash (as my grandparents did back decades ago), you want to do everything at once - first you tear out all the old stuff and then you sequentially have every trade come in to do their respective job.