Cause they're stupid!!!
Some of them are not sorry. Others are afraid of admitting they were wrong, thus opening that big scary door to all the other times they may have been wrong.
People are much more likely to say "I am sorry" (expressing regret, either sincere or insincere) than they are to say "you are sorry" which seems to be an effort to tell you what you are thinking,...
Flowers, chocolate.....
if you have done something wrong you need to prove to that person that you can be trusted, spend time with them, or if they wont let you, leave it for a while let them cool...