I don't think it has anything to do with resources. It's more to do with anyone in the galaxy can decide to end your planet, do you really want to bet your civilization on ALL other civilizations being friendly? It could be rational, game theory, "grabby aliens", religious reasons, etc.

The other issue is you could meet a perfectly sane looking alien civilization, but lose track of them, and before you check in again (say after 100 years) they could be a completely different civilization. Just imagine if the USA can go from far Obama to Trump in a year (or a day), how far you might get in 100 years.

Combine the high latency communications and impossible defense against a good offense (near light speed weapons) leads to the dark forest hypothesis.