If you assume that people are locked into a fixed position in the economy, then yes, those people have seen negative wage growth when adjusted for inflation.

But if you assume that new entrants to the economy develop skills and move up over time as they acquire skills and responsibility, as is likely a closer reflection of reality, then their wages have grown over time. The observation you are making shows that for new entrants to the economy at low income levels, there has not been a net increase in the starting wages those people earn since the 1970s.

I guess the question to ask is: should we expect the real earnings of low level, low skill jobs (not people) to increase over time as the economy does? As you said, these are relatively flat. I have no reason to assume someone performing low-skilled labor in 1970 should be paid more in real terms than someone today doing the same job, in pure economic terms. For that to be true, they'd need to be performing some kind of increased value in their job relative to 40 years ago -- but if they were doing that, they wouldn't be performing a low-skilled job anymore, and hence would be moving up in income level!