In Sports, what causes teams to do so well, great in the regular-season year after year but always come up?

short in the playoffs, the word "Choke" comes to mind, what causes teams in Sports to choke in the post-season, playoffs? Teams such as the Donnovan McNabb-led Philadelphia Eagles of the 2000's, they made it to the NFC Championship game 5 times winning it just once, and we all know what happend to them in the Super Bowl, the San Diego Chargers of the late 2000's, the Dallas Mavericks and Phoenix Suns of the NBA, the San Jose Sharks and Washington Capitals of the NHL, and the Tony Romo-led Dallas Cowboys, the Buffalo Bills of the early 90's, losing those 4 straight Super Bowls, Seriously, what is the point of doing great in the regular-season if you can be great, be successful in the post-season? Don't teams ever learn from their mistakes?