Because you don't expect street signs to be visually pleasing. You expect a certain look from street signs because they all share the same function.
But startups come from various fields and solve various problems, provide different services and products. Should we shift our expectations that all startup sites will use templates now?
Your startup is not a special snowflake. Your buttons and menus and checkboxes work just like everybody else's. Your buttons can look just like everyone else's buttons. You know what your users will say? "Oh! It works just like X, I know X, this is easy!".
But startups come from various fields and solve various problems, provide different services and products. Should we shift our expectations that all startup sites will use templates now?