I have wondered this as well. I have also wondered why Par 5's aren't longer as well. You are correct, no one bats an eye when they see a 500+ yard par 4, but they crap a brick when they see a 260 par 3, or a 700 yard par 5, which I've only seen 2 of in my life.
They are just tough holes to build. Its very difficult to make an interesting 300 (or 275 or even 250) yard par 3. I mean, the green necessarily has to be huge, the approach has to be reasonably flat and even the green has to be somewhat flat. Otherwise the hole can get out of hand.
As far as why aren't holes built as over par Half Pars, they're boring. A 275 yard par 3 or a 525 yard par 4 can't be built with too many features or they get incredibly difficult. It's why 4 at Pebble is better than 2 at Pebble. On that same note, you don't ever see a 2.5 par 3 which would be really cool.