Why do we buy compost?

My garden soil is clay, so using that to sow into would probably not result in very happy seedlings! The seeds and plants I plonked directly into clay last year (out of necessity as I hardly had any compost then) did not do well at all. I'm using bulk compost purchases to get my garden started. After that, I agree that buying lots in isn't ideal. However, unless you are composting everything the garden produces (not eating any!) then you are going to have to bring nutrients and organic matter for drainage in from outside in one form or another.