Well, because the Cutter is a one-time investment, and every run after that makes 70 times the profit. The Cutter isn't gonna pay for itself the first trade run, but after it pays for itself you are gonna be bringing in profits MUCH faster than that Sidewinder is.
It's not a 10,000 to 70 ratio. It's a 10,000 to (70+70+70+70+70+70+70...) ratio.
Right, sure,
after it's paid for itself it's all profit. But it takes a very long time to get to that stage, and it takes much longer to get to that stage in the Cutter rather than the Sidewinder. Certainly the Cutter should cost more than the Sidewinder. The Cutter should certainly cost more than 70x as much as the Sidewinder. But 10,000x as much?
Let's take a simple trade progression: Sidewinder -> Hauler -> T-6 -> Asp -> T-7 -> T-9 -> Cutter. (With the aim of eventually buying a combat-fit Cutter)
Ships are fitted to the Coriolis "shielded trader" template, cargo profit is 3k/tonne, and the old ship gets sold each time. LYR discount of 15% used for purchases. Rounding errors used to pay for refuelling, and assuming NPCs continue to be trivially beatable in the interdiction step so no repairs or rebuys.
Sidewinder: 163,370 credits. 8t cargo.
Hauler: 201,909 credits. 18t cargo. (bought after 2 runs)
T-6: 889,058 credits. 104t cargo. (bought after 12 runs, 14 total runs) - rebuy is a 0.14 run setback, plus a couple more to make back the lost cargo
Asp: 10,476,378 credits. 120t cargo (bought after 30 runs, 44 total runs) - rebuy is a 1.45 run setback, plus a couple more to make back the lost cargo
T-7: 20,200,463 credits. 272t cargo. (bought after 27 runs, 71 total runs) - rebuy is a 1.23 run setback, plus a couple more to make back the lost cargo
T-9: 83,320,375 credits. 500t cargo. (bought after 77 runs, 148 total runs) - rebuy is a 2.77 run setback, plus a couple more to make back the lost cargo
Cutter: 225,919,435 credits. 728 cargo. (bought after 95 runs, 243 total runs) - rebuy is a 5.17 run setback, plus a couple more to make back the lost cargo
Cutter, combat fit: 833,228,386 credits. (bought after 278 runs, 521 total runs) - rebuys are 41 million each (so 19 trade runs after refitting it back for trade)
My opinion is that this is basically going fine until the T-7 purchase, and then after that it really slows down far too much. And this is in fairly ideal conditions - a beginning player probably isn't going to be doing consistent 3k/tonne trade routes.
As an aside: The number of runs a rebuy sets you back also means that the whole "why don't we have the high-profit trade routes in the Anarchy/low sec systems" thing is basically untenable, too - if you need to succeed 9 times out of 10 in the Cutter so that the rebuys don't mean you're actually going backwards - and probably more like 29 times out of 30 so you still make more progress than you would just going somewhere safer - then that's not a dangerous system because if you can succeed 9 times out of 10 then either a marginal increase in skill or outfitting will make you succeed 10 times out of 10, or a marginal increase in enemy danger will mean you're getting beaten out of your rebuy cushion very rapidly and have to head off to do 3 times as many trips somewhere safer instead. It's just not possible to balance the combat danger that specifically.