Because human expression is cool, man. "You guys are killing it!" is way better than "You human beings are performing above average for this task. I appreciate this". Also it's not a weasel word. It's totally just an expression.
I find it truly incredible that someone feels the way you do.
If someone told me I were killing it, I would feel like a 22 year old engineer again and have flashbacks to that one PM who loved my work but who came to work still tripping some Mondays. Glad to be making a key stakeholder happy, but holy fuck am I glad we finally fired that tool.
If someone told me I was doing above average and they appreciated my work, I would feel... appreciated and proud of my work.
The world where killing and crushing are used in an unironic way is tiny and occupied mostly by people under 30. Can you retire at 30?
I find it truly incredible that someone feels the way you do.