I feel like nobody really thinks about why it is this way. While playing the game, I also realized gear is rarely an upgrade on what you already have, and I feel that it's an incentive to take the base gear you find and use the upgrade and materials systems, and thus the outpost system or ship building systems, to take the weapons from mediocre to what you actually want them to be.
If the game just gave you awesome loot on the regular then you'd have absolutely no reason to engage with the upgrades