Pres. Barack Obama is famous lacking empathy
Citation needed.
but also for wanting to appoint judges with empathy
Because it's nice when the law is interpreted in a way that takes actual human beings into account.
and for blaming 9/11 on an absence of empathy in the terrorists.
A profound lack of empathy may not be necessary to sincerely want to murder thousands of otherwise innocent people whom you've never met and have nothing personally against, but it really helps.
I am wondering why everyone assumes that empathy is a good thing.
Because it's what allows us to understand and identify with other people, making civilization itself possible--at least a civilization based on things like mutual cooperation and aid. It helps prevent us from doing things to other people that we ourselves find repugnant when done to us.
In other words, empathy is what marks the difference between an normal, functional human being, and a psychopath.
If psychologists can measure whether it is going up or down, then they ought to be able to measure whether it is making us better or worse off. Where's the evidence?
Contrary to what you'd rather believe, violent conflict around the world is very slowly decreasing rather than increasing, and has been for a number of decades now.
If it is good, then why is it that the upper classes don't want it?
Because it REALLY gets in the way of a society ran exclusively by, and for the sole benefit of, authoritarian psychopathic elites. The people you idolize see other human beings outside of their social class as either a resource to be used and then discarded when they're no longer useful, or as obstacles to eliminate--nothing more.
I suspect that high empathy measurements are correlated with an assortment of psychological problems.
And I suspect that you're a moron.