Why throw away so much sourdough starter? Tartine Book no. 3

this is the first time I've use this site to ask a question:

I've been meaning to bake some sourdough bread for a long while. I've been just hung up on the recipe for the starter. A book I'm currently using for inspiration says to do roughly a 1:1 ratio of water to flour of 300 g warm water and 315 g flour. Eventually, just before feeding the starter, it calls to "Transfer 75 grams of the starter to a clean bowl and discard the remainder of the starter." Later it then asks you to continue this process, discarding the sourdough and leaving 75 grams.

When I first read this I had to blink. Discard 540 grams of sourdough starter?!? What a waste!

Why is this book telling me to do the in the recipe? This seems incredibly wasteful. I don't understand why I couldn't just keep feeding it with a 1:1 ratio.

The book is TARTINE, Book No. 3 By Chad Robertson.