Why are crystal capacitors considered to be in series?
There are two aspects of load capacitance.  What the crystal sees is the capacitance between the two ends of the crystal.  Typically the oscillator circuit will need some capacitance between one end of the crystal and ground, but that's of less importance to the crystal.

If the two ends of the crystal were moving up and down in perfect anti-phase fashion, and two load capacitors were sized according to the inverse ratio of the amplitudes, then current flowing from one capacitor into ground would precisely match current flowing from ground into the other capacitor, such that if one disconnected ground but left the capacitors connected to each other, circuit operation would be unaffected.  In that situation it would be obvious why the series value of the capacitance would matter, because the only capacitance involved would be two capacitors, in series.

In practice the two ends of the crystal don't oscillate quite 180 degrees apart, and the capacitors aren't sized to match the amplitude ratio, so there's a little ground current flowing in the caps, but it's generally only a small portion of the total cap current, so the dominant behavior is still that of the two caps in series.