People say it because they experience similar cliffs in the tax system even with higher incomes.
I live in the UK. Currently my marginal tax rate is something like 65% because for every £100 I earn, I pay 40% income tax, 12% national insurance, 9% student loan (which functions more like a tax here than a real loan), and I lose something called child benefit when I earn more.
So yeah, I want to earn more, but it's pretty marginal returns for the extra work, stress and responsibility until I've totally lost child benefit (lost totally at £80k) and then I start to keep more of the income again. And then at £100k you lose the government support for childcare, you start having to pay interest on all savings accounts, and so between £100k and £120k you can be worse off than before rather than better off, especially if you have multiple children.
That's not to say that the incentives are the same on the low end of the salary scale but you can see why people might think it.
The entire tax regime in the UK is outright designed to inhibit class mobility and penalise hard work. The 60% tax trap and other benefit cliffs over 100k are just punishing. At the same time the capitalist class enjoy a cool 20% haircut on capital returns.