I think that also stems from shell using the stdout as data transfer for both humans and machines at the same time, and people expect to be able to pipe the out

I think that also stems from shell using the stdout as data transfer for both humans and machines at the same time, and people expect to be able to pipe the output into another process for further processing. Extra text intermittently added would make this far less repeatable and lots of extra lines to filter away before processing. Sure, you've got stderr, but most terminals will print these in the same way so a user doesn't know what is output and what is error really. Goes to show how bad such tools are for automation really.