UI can be an important part and not seems so easy

• Creating user interface code is time intensive.  Not just in terms of the initial coding, but in my experience UI code generates the most defects and hence takes the most time to fix bugs.  We can cut down the defects by extending unit testing as far into the user interface code as possible.  TDD opponents often use the UI as an example of code that just can't be tested.  They're wrong, but UI testability doesn't come for free.  It takes some care to craft a structure that makes UI code easier to test.
• User interface code interfaces with an extremely unpredictable external dependency -- those wacky users!  It takes a great deal of care to protect the system from garbage input and user error.
• The User interface changes frequently.  Again, my experience is that it's much easier for a user and analysts to get the backend logic requirements upfront than it is to specify the user interface.  Making the UI code easier to change just makes sense, but that again takes some design cycles.
• UI code is largely event driven, and event driven code can be nasty to debug.  Again, testability and a cleaner separation of concerns makes that debugging either go away or at least become easier.