Because you "asked".
AddOns aren't their to clutter your screen, if anything, they're their to lower the clutter on your screen. As seen by most addons, they tend to get rid of the useless art around for example, minimap, actionbars, unit frames etc. and replace them with simple borders or in some cases (SimpleMinimap) even worse clutter.
AddOns provide you with the ability to choose which information (more, or less, information then the default UI can provide) you want to see and where you want to see it. To some point this is a personal opinion, though I guess most (good) UI developers know what is important and what is not.
A User Interface is their to show information, because of the way we perceive things, the default UI just isn't optimal for it. Does this mean you can't play with the default UI? No. Does this mean playing with a personal UI is better? In some way, yes. Do note that the majority of UI's have little thinking behind them and don't necessarily improve gameplay.
A well known example would be "DON'T STAND IN THE FIRE NOOOOOBBBBB!". As we all know, the character you play is at the middle of your screen, most of the things happening or showing will thus be at the middle of your screen. Making a black frame over your character would thus be unwise, as it blocks crucial vision. Playing with no UI would be ideal, but you simply need information.
Designing pretty AND functional UI's is something not everyone masters. But those that do are guaranteed to play better, if they don't it's simply a design flaw.
Playing with:
My own UI
or
Random default-ish UI
is a massive difference. That's not an opinion.