We can only work on a small set of projects if don’t know a lot of programming languages, because of the great diversity of used languages. So we have to choose between spending a lot of time learning new languages, or restricting the number of projects we can work on.
We have to duplicate most of the tools which help to program in a particular language (debuggers, syntax analyzer, libraries, framework, etc…). Increasing the work to be done, and decreasing the quality of these tools.