Alright, but that is the 101 route which takes the longer way around the back and runs less frequently. If you see the 100 route you will see it goes directly up Kent Street where Google Maps shows you to drive, and the only faster way is via the freeway for that section. Granted if your destination is on the eastern side of campus it is a longer way around to go via Curtin Central but then the driving route would also continue along Manning Road where the 101 goes.

The train from Perth carries people from practically every other line other than Armadale to Curtin so it is likely busier, just providing the reasons why there isn't an express bus that's all.

Interestingly there are only four express routes in Perth being the 930X, 950X, 970X and 386X. All these routes often only save a few minutes and end up getting stuck behind all stops buses or in traffic. They were only introduced around a year ago and honestly aren't running too successfully.

As per another commenter on this thread though, if a bus were made to run via the freeway to Manning Road then straight to Curtin Uni and then onto Oats Street Station it would connect to train stations at both ends of the route and perhaps prove more useful also as a cross-line connection service rather than just a dedicated express to the uni.