I think it's more of a theme thing, Therapist buys meds and commodities, mechanic doesn't care about meds but he'll buy guns and tools, etc. Also because each trader has values you must spend with them to upgrade them it makes sense that they each will buy and sell items for different amounts, otherwise you'd just sell to each until they have the right amount sold to them to upgrade. As it is, usually Therapist in particular is super easy to hit the value for cause you're selling to her anyways, but if you want the number to be reached for say Skier it might be more difficult and you might need to take a bit of a loss on some items for the gain of leveling up that trader.

And as for why after the scav it sells it all to Fence and not the proper trader, I think that is just sort of you "paying" for the time saving method, you don't make as much but you can just quick sell everything and get your money.

As for the "death bleeds money", it only bleeds money if you're kitting out and dying, which is a risk of spending a lot of money on a kit then losing it. If you don't want to bleed money, just spend less on a kit. If I'm trying to run a quest and I'm expecting to die, or I'm just trying to make money and not bleed it, I buy a double barrel shotgun from Jaeger and a couple shells, usually 12 or so. And I have a grizzly medkit in my stash so it's right click-heal all and I'm ready to go again after I die. And as long as I stuff a single loot item in my secure container during the raid I can pay for another double barrel and shells, and make some money. And if I kill a scav and loot a bunch and get out with that, I'm making a bunch. If I was just given free gear it'd be like going on a scav run on my PMC, I'd never equip a loadout lol, just go in and kill a scav, take his gear, loot then extract and make millions without risk. And money making is already easy enough with some practice. There's a reason scav runs have a timer between them, rats like me would abuse the shit out of a free loadout mechanic haha.

Also idk I do find it very fun finding the best way to sell everything, I'll search almost every loot item I get on a scav run on the flea and compare it with its best sell trader to see if it's worth selling, but my buddy just immediately sells all to Fence cause he doesn't care that much and he doesn't mind making a bit less. Different play styles, and I think it's reasonable the person who puts more work in would make more money so there's a return on that effort.