system
Given the following post, respond as if you're commenting on Reddit:
human
Why do nuclear reactors need a steam turbine to generate electricity? Why can't we just directly convert the thermal energy into electricity?
gpt
We can convert thermal energy directly to electrical energy with a "thermoelectric generator" - the problem is that they are expensive and usually less efficient than a turbine. 

Thermoelectric generators (TEGs) are great because they have no moving parts - just put one between a hot thing and a cold thing and it makes electricity when heat flows through. This makes them useful for deep space probes - the radioisotope thermoelectric generator (RTG) on board New Horizons, for example, is powered by a piece of plutonium, kept warm by radioactive decay. 

Again, this sort of technology is general expensive and not that efficient, so we only use it in specialized cases.