Software developers are doing R&D, you create new stuff that continues to exist forever. This is unlike most jobs, in which you produce goods and services that are consumed and then you have to produce them again.
What if we are getting to the point where all of the low-hanging stuff is invented? For example, every bank has an app which works and rarely needs to be changed. Or take a social network like Facebook, going from zero to a product with 1B users took a lot of development but since then, it's been mostly static, why employ thousands of developers dedicated to Facebook? Etc.