Why can't the power divider be used as a high-power combiner?

As shown in the figure:

As a power divider, the signal is input from IN, and when the signal reaches the points A and B, it is the same frequency and same phase signal.
Therefore, no current flows on R, that is, no power loss, and R plays a role in improving isolation here.
The signal is divided equally between out1 and out2.
At this time, the power capacity mainly depends on the power carried by the microstrip line.
Of course, the selection of the R value should also consider the open circuit and short circuit of the out terminal.
In the above figure, as a combiner, the signals are input from out1 and out2 respectively.
Since out1 and out2 are two different signal sources, their frequencies and phases at points A and B are different.
At this time, there is current flowing through R. After theoretical calculation (refer to: Microstrip circuit design principle), the power carried on R is 1/2out1+1/2out2.
If out1 and out2 input 10W power respectively, then R should choose at least a power resistor of more than 10W.
Similarly, the power of out1 and out2 reaching "IN" can only be 1/2out1+1/2out2.
There is a situation where IN=out1+out2 can be established.
The figure below is a schematic diagram of a 100W power amplifier.

If the tubes of amplifier 1 and amplifier 2 are paired with no phase difference, the same excitation signal is amplified.
At points A and B is a 50W signal with the same frequency and phase.
No power loss on R.
So the energy is all synthesized to the C port.
Since the power divider usually adopts a microstrip structure, the balance resistance R generally has a small value.
And the heat dissipation surface is not large enough, so the power divider should not be used for high-power synthesis.
It is recommended to use a 3dB bridge for the synthesis of two high-power carrier signals.
Since the 3dB bridge can adopt a cavity structure and can use a high-power external load, it is suitable for high-power signal synthesis.