For a number of reasons:
We should never recommend extra steps that aren't necessary to solve the problem. Concision is important.
There is a finite number of PPAs that a user can add (more accurately GPG Keys). If they don't need a PPA to solve the problem, don't make them use up one of their slots.
PPAs are potentially dangerous. Training folks that aren't knowledgeable to enter in Terminal commands that require root priveleges and add PPAs that are maintained by unverified (read: potentially malicious) authors is not a thing we should be doing.