Reverse core dumps are possible, so I'm not sure this analogy is perfectly appropriate.

It makes you wonder if our brains work as RAM or as a more long-term storage when "rebooted".

I assume OP wants to know why we can't provide electrical activity manually.

So in other words it's like RAM, just the power off and all is lost.

Nah, there have been cases with short temporary flat EEG that came back to life, which is why the guidelines specify the duration for declaring someone dead in this way. Those people suffered no memory or identity loss, so electric activity is not related. Connectome structure and tuned bioelectrical responses based on cell cytoplasm gradients, receptor structure and number tuned by a bunch of either RNA or proteins the most likely candidate. So the better analogy is it's like flash memory, left unpowered but running for too long the neurons will change.

Restarting the brain from that flatlined state is therefore possible, we just don't know how to do that properly. Most importantly, it won't do any good if the brain is damaged and is rarely useful.