Because you have a tonne of work to do to make your startup succesfull and dedicating time to re-inventing a wheel will detract you from it. It also means you

Because you have a tonne of work to do to make your startup succesfull and dedicating time to re-inventing a wheel will detract you from it. It also means you waste future-you's time too. For example when you want to integrate apps into your books (sheets is not reliable at scale), when you want to train a junior accountant to look after your books. When you want have to send a copy of financial statements to banks/tax-offices/investors whatever and they are used to a particular format.