a) Those who do have the source code to share it. Sometimes the source code is available but it can still not be freely used and/or shared.

b) Allow modifications and redistribution of the binary artefacts, which for is sufficient for many goals.

c) Remove all concerns with reverse engineering and allow e.g. decompiling programs and sharing that source.

Also, remember that the GPL already does not make source available externally if the modfications are only used internally.