Why don't people who give good advice take their own advice?

It is far easier to be objective when you are on the outside looking in without all the emotions playing.Why don't people who give good advice take their own advice?
An advice is mostly given to someone whom is is need of one ,or ask for it, the person who gives the advise usually has more knowledge in life, and speaks from experience either lived or previously seen in others, knowledge usually applies to an advice given, but the recipient of the advice is free to listen to it or not. Therefore the person who gives the advise draws back and let the the other person make their own decision,Why don't people who give good advice take their own advice?
It's a case of: One rule for them, one rule for me. Or: Don't do as I do, do as I say. Or: Everyone's out of step except me.
I really don't know... i'm guilty of this... i give advice to friends, and when i'm in the same situation i just can't take my own advice it's annoying isn't it? It's probably a self destructive thing...
Because they are not in the same situation with the person whom they have given their advice.
Oftentimes they do; it's just that the recipient is too thick to realize that.
Because they don't think they need it.
Free advice is worth precisely what you paid for it.