Well, it could be implemented like this, it's just more cumbersome than "create N-1 anonymous pipes, fork N processes, wait for the last process to finish": at the very least you'll need to select() on the last unattached pipe, and when job control comes into the picture, you'd really would like the "setting up the pipeline" and "monitoring the pipeline's execution" parts to be disentangled.

Not even that they might be particularly slow to start in absolute terms, but just that they might be slow relative to how fast the previous stage starts cranking out some input for it.

(Since, as GP said, not an infinite buffer.)