To be honest they get quite a few columns done by academics every now and then. A lot do so to sell their book but there a few semi regulars, Evengy Morosov comes to mind. They tend to be a little buried in the fluff though. Obviously dont generate enough click through.
But in all honesty most academics that are successful enough to write for the guardian (and be considered academics) are busy people. The £300 odd writing fee they give to freelancers is not going to make it worth their time anyway. Sure plenty of people with PHDs or who dont have tenure or enough success to go on the lecture circuit could do with that cash, but I mean plenty of journalists have PHDs or came from academia, it doesnt instantly make them more thorougher and insightful writers.
I would just be happy if newspapers still invested and promoted actual journalism. Not just quickly written opinion pieces or reporting what someone said on twitter. I mean foreign correspondences have never been so important with the level of misinformation about yet they are almost completely dead in modern news, and rarely ever publish front page or feature length content.