Why?  Because even simple UI’s are complex.  There’s:

• The need to understand who’s using your product
• The need to understand what users are trying to do.
• The need to understand what scenarios the UI will handle
• The concept brainstorming stage
• the concept fleshing out stage
• interactive prototyping
• Feedback throughout the design process
• The front-end code
• The back-end code
• The speed of the app
• The layout
• Responsiveness
• The copy
• Typography
• Colors
• The page hierarchy
• The interactions
• White space
• Motion
• Personality to convey
• A story to tell
• Feedback states
• Error states
• Accessibility
• Small delightful details
• Internationalization
• Retina considerations
• Considerations for decisions vs. options
• Browser testing
• Device testing
• The brand to consider
• What did I miss?

When designs are rushed to ship, for whatever reason, chances are some of these items will fall through the cracks.  Bottom line: great UI designs take time to craft. Great UI’s can’t be rushed.