Knowing, or trying to figure out, your life purpose or purposes is hard enough there’s no need to lead a lot of people, possibly most people, down the wrong path from the start by insinuating only one purpose was possible. Also, even if someone had or wanted only one purpose in life, it might be easier for them to identifying it by recognizing a common theme among multiple, specific, likely actionable, purposes more obvious to see initially. That is, they might see the whys from the whats of their actions more easily if those whats were just accepted as multiple whys without initial judgment of being wrong.
Another problem with having a singular life purpose is that it can either reduce your life to something narrow, if the purpose were specific enough to be actionable, or be so vague as to be impractical, needing more specific details that are essentially multiple life purposes. So says the guy with a single life purpose of For Good in his life strategy, and a dozen supporting life purposes as to how he will live his life for good, to uphold the overarching purpose he derived from the more actionable purposes.
Finally, life purpose can change with time. As life foci changes, past foci might be discarded or reprioritized. To force the issue and keep just one life purpose only makes a difficult task more challenging. So when talking about purposes in life, start with the plural, and don’t be afraid to leave it there if you can’t bring it down to the singular.