Because the guys who write marketing copy have nothing to do with managing the DL service experience, which obviously fluctuates like crazy while management does nothing about it. See this thread, "Fear and Loathing at LAX," for an interesting real-world counterpoint to all this talk of great service.

It all reminds me of the UA "Rising" campaign that broke around 2000 amid horrible labor disputes and, eventually, UA's Summer From Hell. The marketing message was so wholly disconnected from the actual awful UA experience, they shouldn't have said anything at all.