Because Netflix, unlike say HBO, bills themselves as a general source of entertainment, a place you can always go when you're bored to find new content. While somewhat true in the beginning Netflix today is increasingly leaning on original content because most other big players realized that they could cut out the middle man and still be profitable. Netflix's library today is basically their original content along with things you could find in the bargain bin at your local Best Buy which is a much worse value proposition than "your one stop shop for movies and TV". So their UI is designed to make their library appear larger than it really is.

Because their library gets smaller and smaller every year especially their library of non-Netflix produced content.