Why are so many Jews atheist/agnostic?

Jews are not atheist, by definition, unless your definition of 'atheist' is anyone who doesn't believe as you do yourself.

As for agnosticism, Judaism tends to be atheistic. Jews don't believe there is only one interpretation of every phrase of scripture. When you study Judaism you study the thoughts and commentaries and exegetics of great Jewish thinkers who have lived over the past several millennia. And they don't all agree on everything! Their differences are -celebrated-, not swept under the rug. Jews tend to be freethinkers!

I think its because a lot of them break their religion down to biological and physical states of reference while religious ideas are about going beyond that to the invisible world. Also because their religion does not eschew worldly pleasure the way christianity or buddhism does.

A long history of critical thinking.

They are educated and intelligent.

They are Khazars.