Almost every program that we run has access to the environment, so nothing stops them from curling our credentials to some nefarious server.
Why don't we put credentials in files and then pass them to the programs that need them? Maybe coupled with some mechanism that prevents executables from reading any random file except those approved.
Environment variables can be set individually for each app, no need to set them globally. If you put credentials on files, that file can be read by any program also if you don't isolate it, as you should do with environment variables too.