It's supply and demand. Scientists publish their discoveries to the commons, so there isn't much demand for people to hire them. Many would-be scientists go into fields like finance and engineering specifically just for the pay (fields that are in demand, but have low supply). Science is a public good, so a market failure occurs.

"Unskilled labor" is labor that many can do or learn on the job, so there is a high supply. It doesn't matter how hard or essential the work is, it's going to be low pay due to the low barrier of entry. Which is unjust, and Why Socialism (Einstein) is needed.