If I had to bet, they have those systems but they are a pain to use. If I had to speculate, it would be a lot like those citation search sites you used in College, they simply didn’t have the usability of Google. You had to enter search terms very specifically, whereas with Google you just search for what you want and Google figures out how to structure the query best to give you the results you want.

I can totally see a scenario where a clerk needs to look something up on a Friday afternoon, the though of pulling up the proper search catalog pains them so they shortcut, googling the thing they are looking for a reading the citations on Wikipedia. If I was a law clerk I would 100% do this.