Margins and as many wafers as they want, eventually. If they can get their 7nm process working they will have a lower cost and higher capacity than their competitors since TSMC does exist to make money and other companies want their advanced nodes too.

Also there's a reason Intel's 14nm chips can compete with AMD's 7nm chips. Manufacturing and design working together has advantages of its own.

Finally it seems like they plan to make some chips at TSMC.