There's actually several aspects here, which make it reasonable that combat pays worse than other activities. I'll limit myself to the top three.
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1. Fun
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Many players consider combat to be the most fun activity in the game. That means that it's the most common source of income. So to make other careers viable, at least on paper, they need to have higher revenue than combat.
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2. Risk vs. Reward
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It might sound crazy, but I consider combat the lowest risk activity in the game by now. When I go mining trading, I use a ship which has moderate shields, no HPRs or MRPs and a very limited selection of defensive modules. After all, I need the cargo space. When I go mining, the same is true, except that some cargo space is instead used up by a refinery and limpet controllers and I also carry a reduced weapon load, to make space for mining lasers. For exploring, my ship again replaces defensive options with ADS/DDS, SRVs, heat sinks, etc.
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Most of my combat ships feel like you can detonate a nuke right next to them and it would merely inconvenience them a little. So thanks to insane defense stacking, engineers and parts of the community successfully crying away any rebalancing attempt, combat actually is the lowest risk activity in the game by now. Thus it unfortunately makes sense that it also pays worse than other activities.
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3. In game logic
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This one actually is related to #2, but transfers into the game. If there's combat and pirates all over the place, then non-combat activities are logically in higher demand and thus more profitable. It really boils down to the picture of 500 heavily armed but starving soldiers in the desert fort, awaiting the enemy to attack. Now what do they crave more, the 50 men reinforcements, or the food caravan? (And what would be more painful if the enemy intercepts on the way? )
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In the world of this game, it really makes sense that just killing a criminal doesn't pay well. There seems to be an endless supply of those, after all. Escorting a convoy of goods to a destination, that's what should be lucrative. Unfortunately FD seems to love the idea of that, but doesn't seem to be able to really support that. They tried with wings, but the small payout for escorting a transport is not cutting it, either.
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Some escort missions would go a long way, but with supercruise and how interdiction works, it's really not that easy to find a way to do it, which would not appear broken right from the start.
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