Why are people so mean ?

Answer 1

1. This question is ludicrous, because:
a) you have just stereotyped the whole human race, including yourself;
b) there is more than one significance of the word "mean" (petty-minded, selfish, spiteful, stingy etc.); and
c) you have given no context to your assertion.

2. That is why the question can not be answered competently.