Let me clarify that when I say a large sunk cost, I mean it as in you will generally need to obtain a large fleet in the first place. Lets say that you have loads of combat skills a lvl 20 officer and 2 Paragons. You still want to deploy them together so you need a fleet large enough outmatch the bounty fleet so you have 120 DP and they don't. How many other capital ships and cruisers do you need in order to do that? Or perhaps you are happy with a Paragon and whatever else you can fit into the DP. You'll still need a comparable amount of capital ships and cruisers. If by doing bounties, it takes a long time to get to that level of fleet and you are very suspectable to downtime. Or perhaps you'll deploy a single harbinger over and over again. Multiple times. By the time that battle ended, you could had gone from a wolf start to doing 100k+ bar and procurement missions.
Meanwhile by way of comparison, by doing bar and procurement missions, which in the case of procurement mission, will not require more than 500 cargo space which is easily achieved naturally or just by buying a cargo destroyer. It's not risk free, but you will hardly ever have to wait around for a mission appropriate for your fleet, and you will almost always know that it completable profitable, enabling you to save and reload if something goes wrong. Completing the mission will take 1-2 minutes. Meanwhile bounty mission forces you to go out somehwere, where if you are unlucky, you might goto the wrong star or head to the wrong world, for even more travel time. And then you got to fight. And then head back.
As a way of making money, bounties are less effective than bar and procurement. In order to make money, combat is seen as something to avoid, which is the opposite of how it should be, which is a shame. This game is best when you are fighting. I know people demand to be able to trade profitably, but the focus on the game should be about combat, not about making money. Because once you can make money, there isn't much you can do enjoyable with it. Other than combat. And then you have to find reasons for combat, because you certainly don't need the money.
I suppose in theory trading can be fun, dodging pirates hiding in nebula and the like, but I'm guessing most people just press shift and emergency/sustained burn on and reload as the fastest way to make money as it is now.