How do you know people won't want it unless it's built? I understand not doing it if you're offered equity and you think the idea sucks, but if you're just bein

How do you know people won't want it unless it's built? I understand not doing it if you're offered equity and you think the idea sucks, but if you're just being paid to develop a site, even if its a bad idea, why do people refuse that? This sounds like graphic designers I know who refuse clients because the client wants something ugly.