Because they don't offer to save your passwords the breach can always just be if someone attacks your personal computer, which is so much less likely than that

Because they don't offer to save your passwords the breach can always just be if someone attacks your personal computer, which is so much less likely than that someone tries to attack a company which hosts thousands or even millions password databases. In the open source world you have at least the possibility that someone who you trust looks at the code, with the closed source you don't.