First, because it's very difficult to design an inverter that takes 20 kV DC in and produces 5V out. There are no semiconductors that can take that voltage. This is one of the reasons why DC is not as easy to work with as the AC. For the latter you only need a transformer.
Second, there isn't that much of energy to bother with. Say, the car to ground capacitance is 1000 pF, and the voltage is 20 kV (any more and it will leak out, through tires or through ions in the air.) Then:
u = 0.5 * 1nF * (20kV)^2 = 0.2J
0.2 joules is, by definition, 200 mA through 1 Ohm for 1 second. In other words, it generates 0.2 watts for one second. This is just enough to power a keyfob LED (20 mW) for ten seconds - something that a cheap coin battery does 100x more reliably and more efficiently.
First, because it’s very difficult to design an inverter that takes 20 kV DC in and produces 5V out. There are no semiconductors that can take that voltage.....You seem knowladgible. Get to work. Ask for a grant.