Because it looks cool...
Older posters will remember the early days of Apple computers, when everyone discovered computer publishing for the first time. Every damn newsletter and Christmas card has about forty fonts, squigglies, and other special effects rendering it almost unreadable.
The first days of the Internet had similar bad web pages, with blinking words, wiggling GIFs, and pretty much anything they could think of to render you blind. The worst was one with black type on a red background.