Life is a thermodynamically dissipative structure, which means that it cannot persist without having both inputs and outputs at the same time. It gains the capacity to do work by transforming the oxidised inputs into the reduced outputs.

Without photosynthesis, the only available inputs are the stuff the planet already has on it, which is almost entirely already electronically reduced.

See the Wikipedia page on redox chemistry:

https://en.wikipedia.org/wiki/Redox