Why do movies show CGI stunts that are physically impossible?

I'm not talking about fantasy films but car chase scenes which show cars doing things that are obviously physically impossible. Shouldn't there be advisors or a computer model to make sure what is shown to happen could actually physically happen?
For example you couldn't tell if the scenes in the recent Bond Film Skyfall were CGI or actual stunts. They were amazing because you knew they were almost impossible but still within the bounds of physics. Watching Bruce Willis surfing on a CGI fighter jet or Pierce Brosnan surfing on a giant Tsunami of melted water was about as entertaining as watching a cartoon because you knew that it was entirely CGI and was completely unfeasable. Same goes for the Matrix bullet dodging scenes. I had no problems with any of the creatures in Lord of the Rings but what let it down was seeing half a stone arch collapse, leaving the other half standing, something which anyone who knows anything about how arches work will realise can't happen.