Why do some people use lingo such as chick, dude, etc.? Quite candidly, whatever happened to normal speech?

You answered your own question. Why did YOU say "lingo" instead of language? Because you thought it was cool, or expressed something more than the word "language." Because we'd rather be interesting than boring, and one way to be interesting is to say things in a new way. Because it's easier to say "lingo" than language, just as it's easier to say "dude" than young man. Because it's not an exact synonym, either--every dude may be a young man, or at least express some qualities of a young man, but not every young man is a dude.

What's normal?
The English language is constantly developing and changing. If it wasn't we'd all still be using thous and thees to talk to each other, and saying can not instead of can't. There are words and phrases in Shakespeare's plays (The "beast with two backs" in Othello springs to mind or Iago's reference to Bianca as a "polecat", "I fancied her" often seen as a modern phrase is also Shakespearian) which would essentially have been considered the slang of their day. Language is a living thing, it changes and develops with the times, people in different areas, from different backgrounds personalise it. Words in regional English are unique: the Midlands English "mardy" has no real equivalent in standard English (closest I can think of is "sulky"), in some areas a bread roll is a bap, in some a cob, in some, just a roll. Likewise a woman can become a bird, a chick, a broad, and a man a bloke, a guy, a dude, a fella- whilst these might not be appropriate for formal language, such as journalistic articles, or most written communication, they are a natural way of speaking for many people, part of their heritage, personality and background, and should be celebrated as part of their individuality.
I for one get a bit annoyed with people telling me the way I was brought up to pronounce the word "bath" (with the "a" to rhyme with the a in cat.) is supposedly wrong and I should be saying "barth", it feels like a personal attack, a suggestion that my background is somehow inferior. Language is human, a tool we use to express ourselves, and as we develop as a species, so does our language. There's something about "chick" and "dude" that express something subtley different from just "girl" and "boy", and as such they are useful and expressive words.

bevause people today are allways trying to invent a new way to be cool...they are allways trying to act like someone or something they are not.

My fine fellow, why ever do you think I would speak in such a horrid, wretched and vile tounge, when I can express myself in much more refined ways!

dude

I think it has to do with teenage rebelliousness- if you think about it, most of these phrases are invented and used by kids at an age where they are trying to create an identity separate from their parents. What better way than to make up your own language?
And we've all done it-Think of "Yowser" or "Peachykeen"or "Cool, man".
As far as who uses it, only people under the age of 25 can use the current slang without sounding like an complete idiot.