Why are oscillators 'always' AC coupled with 1000pF?
The crystal's output capacitance is 1000pF and the maximum input capacitance it can drive is 10pF.

Capacitor impedance is: \$ Z_{capacitor} = \frac{1}{j \omega C }\$

In other words, the larger the capacitance the lower the impedance (the opposite of a resistor's resistance-impedance relationship, and the opposite of an inductor's inductance-impedance relationship).

Therefore, the output impedance is 100x lower than the input impedance. Sound familiar?

As for the 10pF, I think it's just a round number they chose as a standard. Similar to how 1MOhm is used on oscilloscopes probes.