In the golden age of comercial software you still could get the source, however it was part of the commercial version not the shareware/demoware one, or as additional option.

Naturally with a license that only allowed its use on the context of understanding the product, e.g. source code for the C and C++ libraries of a compiler for use in debugging sessions.

Corporations legal departments just found out a way to use non-copyleft licenses for the 2nd coming of shareware/demoware, while cutting down development costs in the process.