If a new customer can't find the signup link they might close the tab and never visit again. An existing customer will either know where the login link is or be more likely to try a little harder to find it. So the most valuable real estate on the page is used to convert new customers.

It's the same reason why someuniversity.edu has a big Admissions link and a small login button for current students who are actually consuming the service. The website serves as a billboard more than as a production floor.