> Because the first group has a very low income
That's the incentive to work hard so that you are not part of that group. That group doesn't have much to offer, and often didn't work hard to offer anything society valued. I knew, from an early age, that the consequence of not trying at school was ending up in a dead-end job.
> Because the first group usually works hard and shows effort, while the others don't
Citation needed - their work is hard because they have little leverage. Some have the deck stacked against them, fair enough, but that first group exists in places with less opportunity inequality, and far more welfare than in the US.
Many people spend most of their lives on their careers, trying to improve their worth to society, and offer things deemed valuable. Few go into accounting for the love of it, but because society desires accountants.
EDIT: there are exceptions. Temporary jobs held by students, people from impoverished backgrounds, or who were never motivated to succeed, low-skilled immigrants that fall into these last two categories. These are exceptional, sympathetic situations (except students, their condition is temporary), but society should seek to reduce these groups, rather than normalise them by developing some kind of coping mechanism. Welfare is a good safety net for all citizens, but low-paid work is too. The problem is groups who are stuck in ow-paid work with little opportunity to escape; in those cases it's not "greedy" corporations to do anything - it's exclusively the governments role to participate in any social engineering efforts.