It can often take a while for a TV show to really find its feet, settle into its groove, and become the series that will eventually be known as a classic.
Sometimes it needs a few episodes: BoJack Horseman, for example, took around half a season to really get going. Others need longer: Parks & Recreation and The Office (U.S.) didn't properly click until Season 2, and it was even longer for Seinfeld, despite it being one of the all-time greats.
For other shows, however, time is a luxury they can't afford. First impressions matter when it comes to television, with pilots being used to sell a show to networks long before audiences will even have a chance of seeing it. Get it wrong and your series could be dead on arrival; get it right, and you might well have an instant hit - or set an impossible standard you can never quite live up to.
A pilot needs to stand on its own two feet, but at the same time provide an entry point into a larger story; it needs to be accessible, but set things up; it needs to establish characters, settings, and storylines, but can't give everything away. It's a delicate balancing act, and one that these shows have managed better than any others.