Because Princess crew are, in my experience, some of the best. There is no need to do anything but be pleasant, cheerful and enjoy their company to get exceptional service. I routinely get treated like this yet my upfront tipping is zero and my end of cruise additional tipping is usually modest.
I am a firm believer that tipping is a very personal affair. But I want to pipe in that in no way is tipping large sums of money either at the beginning or end of a cruise a requirement for exceptional service on Princess. That said I have occasionally tipped triple digits at the END of a long sailing when service was absolutely above and beyond the job title description.