Why is it necessary to use so much butter in the crust? Can this be reduced?

Butter coats the flour and prevents the absorption of water and the development of gluten. It is acting as a 'shortening.' You've seen Crisco, all vegetable 'shortening' I'm sure. Well, butter is serving the same function. Without shortening, dough becomes starchy and elastic and not crisp and crumbly.

The classic French pate brisee/pate sucree ratio is 2:1 flour to butter. If you change this materially, you'll simply have a tough tart dough. Sorry.

A couple reasons: It serves as the crucial binder in this no-bake crust (without enough, the whole thing would fall apart). And it's delicious!