There are several theories regarding how a batter should approach a T20 inning. Some prefer to go all-out, attacking from the first ball till the last, while some prefer to take their time and construct a long inning to stay till the end. The common point in any T20 batting theory is not to waste balls and keep the scoring rate up.
Since we are in the early phase of the T20 cricket and the format is still evolving, teams still prefer to keep it safe from one end and attack from the other. This is the reason why anchors are very valuable for any T20 side. But even anchors have to try and keep up with the scoring rate and not apply too much pressure on the rest of the batters.
Kane Williamson is one of the high-quality batters in the world. He's part of the modern fab four that includes the likes of Virat Kohli, Joe Root, and Steve Smith. The common point of all these great batters is that all are not more than anchors for a T20 side. They are too good batters to freely play in the T20 format.