Probably because the P4s didn't come out until a couple of months ago....
I have gone through 3 sets of tires, the factory Mez Z8 and then switched to the Michelin PR3 B spec and the lasted much better but still only maxed out at 13,000km which would be about 8000 miles
check out k1600 forum too for more feedback, but most folks run P3's or Roadsmart 2's; the latter is my choice but I haven't tried, nor felt the need to, the Pilots; if you do, P4's might be better if you do more 2up riding
I'm nursing my Metzlers through 6K. Up'ed tire pressure to 50/50 and seems to help with the cupping. I used to get 12K+ on my much heavier K-LT on Metzlers ME 880. I don't ride hard Going to the PR-4's as soon as they come in.
Watch those metzlers close, mine went from perfect looking to showing cords in less than 75 miles. (3874 miles total)
After 5k miles the Stones that came on my GTL were shot so I went to PR3s the first set went 12k miles the next set went 9k miles. I was going to get a set of PR4GTs but they were on back order so I put a set of Perilli Angel GTs. I've only put 1500 miles on them but so far they are by far the quietest ,softest and best handling tires I've ever had on a bike. Time will tell as to wear.