... As for you not buying off anyone who's bred a suck lamb, I think there's not too great a chance your sheep don't have at least one of those in their ancestry somewhere. And, of course, I'm not saying any breed of sheep ought to require help lambing nor mothering, I don't think anyone's espousing that; this is more a discussion of why a motherless lamb might be chosen to be kept alive rather than culled outright.
Many lambs aren't on the bottle because they failed to stand or suckle, in fact I'd surmise that possibly the majority of those particular lambs fail to make it to adulthood. Quite often the mother was lost or something else happened that was not the lamb's fault, as Brucklay noted. The particular lamb I'm raising now came down with paralysis tick poisoning, neither her nor her mother's fault, nor even a congenital weakness. In fact she's by far the best animal that flock's produced. I am aware being a bottle baby puts her behind though.
... Lost mothering instinct can be restored, with kelp and patience.
Not that I blame anyone who doesn't take that path. More effort, who needs it...? I know it's not actually an option for many farmers.
I very much doubt any of my sheep were suck lambs - I get to know the people I buy off pretty well before I buy. One of them is in much the same boat as me (grazier, no farm to speak of) and has to sell bottle lambs before they get too big for his garden shed, much as I do, the other runs some 1,500 ewes and employs nobody else full time - I cant
guarantee none of his are suck lambs, but I severely doubt it. Of course I would keep a motherless lamb alive - until it is fat enough to send on.
Ticks aside - when the mother has dies for some reason that wasn't the lambs fault - if it is a disease, there is a strong chance that the rest of the flock were exposed to it, but did not die. Id much rather keep lambs out of those ewes than the one that did.
I run a pretty small flock of 300 ewes and I dont really have time for kelp and patience, no.