Why do kids (teens) spell like this?

i'm 15 years old and in my spell like a 5 year old For exampele my friend sent me a message yesterday saying "dhat drEsS @ Dha maLl wuz sho friCkInq fabewouS yhu shouda TrYed iT uhn yu'd beE ahmaZeedd <3 ;)" I mean i know this girl is very smart and she cAN spell but why does she have to spell like that for it takes more time to write/type like that. I personaly think its annoying, and its not just her that spells like that MANY of teens spell like that but Whyy?!?!?!?!?