Why do computer scientists on the whole work under the assumption that P ≠ NP?
As a rule of thumb, for any unsolved problem people tend to conjecture the statement that starts with a universal quantifier - since if it started with an existential one, then one would expect to have a solution found.
Other than this, this topic has been discussed at several other places, see https://en.wikipedia.org/wiki/P_versus_NP_problem#Reasons_to_believe_P_.E2.89.A0_NP or https://rjlipton.wordpress.com/conventional-wisdom-and-pnp/.

Update: Or the very recent Chapter 3 here: http://www.scottaaronson.com/papers/pnp.pdf