Why do heat pumps use a separate “reversing” signal from thermostats?

Think of it as a double ended solenoid once it moves it stays there. O moves the valve from heat to cool and it stays there. B moves the valve from cooling to heating.

This is better than having a single spring loaded valve because a minor power bump will dump the pressure in the loop and that would be hard on the entire system coils and lines included.

With the valves requiring force of a solenoid to move them they stay in heat or cool. Many systems require a 3-7 minute off period for the compressor so there is not pressure in the loop to make the change because it’s that hard on the system.

Think of it this way one side may be close to 400 psi depending on refrigerant the other side 100 so there may be a 300 psi thump for a power fault on a spring loaded valve, now it makes sense why they want to force them each way allowing the pressure to equalize and not have the thump.

This is because control boards and programmable thermostats are recent inventions

The concept of a "control board" in a piece of HVAC gear is a fairly recent invention, dating back to the 1980s or so when single-chip microcontrollers started to become inexpensive/widely available. Before then, if you wanted anything more intelligent than direct control of the various pieces of equipment (burner or element, fan, compressor, reversing valve), you had to do it the hard way with relays, sensing switches, and dedicated timer boards, such as found in the fan/limit system of a pre-control-board gas furnace, or the dedicated timer defrost controls in early heat pump systems. Furthermore, programmable thermostats as we know them today also aren't any older than control boards, while heat pumps and heat pump thermostats date back to well before we had any of this solid-state stuff roaming around.

As a result, what you had with early heat pumps was a bimetal thermostat with extra contacts on its mode switches sending control signals directly to the compressor contactor and reversing valve on the heat pump's outdoor unit, or to a dedicated defrost timer/relay that could override the thermostat to initiate defrost. This saved money on extra relays in the outdoor unit at the cost of requiring (cheaper) switch contacts in the heat pump thermostat, and a slight bit of extra wiring in the walls.

Furthermore, the use of a separate signal allowed the reversing valve to be kept in the same "idle" state across cycles during both heating and cooling operation without further logic inside the outdoor unit, as the O/B signal's state does not depend on whether the thermostat is commanding a cycle or not. This is important because reversing, while simple to implement, puts non-trivial stresses on the rest of the heat pump, so you don't want to "swap ends" unnecessarily. Unfortunately, some zone boards exploit this behavior by require both an O and a B wire, which makes installing a modern thermostat on them a non-trivial proposition.