Because it's ambiguous. It's up to the officer to decide. They've rejected common law applications for being apart for 2.5-3 weeks, for example. Also the poster said 2 months, not 3 weeks.
Right, but since you answered my question "1-2 weeks", I'm asking why is it a problem in 3 weeks vacation.
I think this is a case of: do you want to be right or do you want to be approved? Just because you know it's a vacation, and just because you know you fully intend to reside in Canada permanently, does not mean IRCC will necessarily see it that way. There's no hard and fast rule because it's going to be up to the judgement of whatever officer you get. In the original example, I could see how spending 2 months in another country with your spouse (albeit for the perfectly reasonable situation of visiting your newborn child) could be seen as you attempting to sponsor from abroad as a PR.