Question

why is it physically reasonable to expect that two identical parallel plate capacitors that are placed in series ought to have half the capacitance as one capacitor

Answer 1

In series
the charge has to be the same on both, call it Q
by definition each capacitance C= Q/V
so Veach = Q/C
now in series the voltages on each capacitor add so
V total = 2 Q/C
but by definition our equivalent capacitance = Q/Vtotal = C/2

Answer 2

Use the same argument but same voltage on each