Why so high? ( 2 pipe Steam/vacuum system)

I don't know the exact system and I'm sure you probably know @LFsuper , but the Empire Stat building runs its pressure at about 2.5lbs. Not at all familiar with your system, but steam rules even at low pressure for very tall buildings.

Seems it would make more sense to at least partially modulate the firing rates rather than maintain that much pressure and control the steam with a motorized valve....................

Another reason i understand for why the dunham-varivac system is set up this way is the idea that the vacuum stretches out the steam. Since it is moving in a vacuum it remains steam longer because in a vacuum you will have steam at less than 212 degrees. When you have steam at 180 degrees or less you have less condensate and a very efficient system.
I have never worked on a new one, only old ones and I leave them they way they are.
I work on one 27 story vacuum systems (not dunham) where I operate at 2 pounds or less.

Does the steam have to lift the condensate anywhere at all?

It needs 7 PSI "because I said so" and don't question my knowledge......so there!

Boiler pressure is a very complex subject. But, it cannot simply be reduced to "Pressure is bad."
Everything that I understood from my prior experience with pressure steam went out the window when I was confronted with a residential vapor steam system. I had to learn a whole new set of rules. Residential steam systems are designed around the premise that the maximum pressure is 2 psi. However, that amount of pressure is almost never required and seldom occurs in systems where the boiler is properly matched to the load, which means that the system must be able to function properly with zero pressure. Huh? (scratches head)

We all know that it is so important to keep the pressure down. Dan has so many examples of systems not working. He gets called for a consult, pulls out his screwdriver and turns the pressure down, and everything starts to work just fine. And so, it is so important to keep steam systems operating within the parameters in they were designed, which usually, but not always means turning the pressure down.

Energy is neither created nor destroyed - a basic law of physics.
But often times we get a little confused because we can't actually see what is going on inside of those pipes and boilers, although we all paint little pictures in our heads of what we think is happening . We tend to erroneously picture a boiler running under pressure as if it were an air compressor and a pressure tank. The higher the pressure builds in the tank, the more energy is required to pump the air into the tank. If we can get the air to move down a 100 ft long hose at a half a pound of pressure, why would we waste all that energy pumping it up to 60 psi? We visualize that energy used in building boiler pressure is lost or wasted, or perhaps destroyed. But... while mental pictures help us to understand, they sometimes cause us to misunderstand.

Boilers that run under significant pressure are most typically of much higher water content than a residential boiler, so lets think about a small scotch marine boiler with 200-300 gallons of water. (it makes it easier to visualize.) As the boiler is running and building pressure, something else happens at the same time, the temperature goes up---both the temperature of the steam and that of the water. At 10 PSI, the temperature will be about 240 F. If a boiler is set to cutout at 10 psi on back in at 5 PSI, the burner will shut off when the pressure reaches 10 PSI. If there is demand for steam somewhere in the connected system, steam will be flowing out of the boiler and as it does so, the water in the boiler will continue to boil. As this happens, the temperature of the water in the boiler falls in relationship to the amount of steam that has been produced in the boiler and used somewhere in the system. This boiling, and dropping of both pressure and temperature will continue until the pressure falls to 5 PSI, at which point the burner will start again. So, the amount of steam that comes out of a boiler that is under pressure is far more than an equivalent amount of air in a pressure tank. In fact, ALL of the energy that went into the boiler water will come back out in the form of steam. Its that "change of state" magic of steam.

Now, the picture I painted above assumes a perfect world, which never happens. The inefficiencies come from standby losses and boiler and piping losses, which equal wasted heat. The higher the pressure, the higher the temperature, and thus the greater the losses, which are mainly wasted heat to that HOT boiler room.

When comparing a VariVac system to a conventional steam system in which the building temperature is controlled by intermittent firing, the added efficiencies of the VariVac system more than offset any inefficiencies in the boiler room. It gives a much more even and controllable means of getting the right amount of heat to all parts of a building. At the same time, the moderate temperature of the piping reduces losses to areas that do not need to be heated and are overheated by intermittent steam systems.

My descriptions above are dependent on well designed boilers and piping that are properly controlled and insulated, also dependent on good engineering. I have seen many examples of both good and bad engineering, as well as outright engineering errors. They can be nightmares to the people charged with operating a building.