Firstly, I believe it's because it's usually one of the first tools they pick up. Pressure, grain direction, angle of attack are all things that are new to the student. However I think the most important thing is realising that accurate marking out is the foundation of every subsequent process you go through. I always tell my students:
“It doesn't matter how accurate your sawing or chiselling is if you're working to inaccurate lines.”
I think that many of my students rush into things too quickly in hopes of getting into the stuff that involves making some mess. In doing so, neglecting the stage that should take the most amount of time.
In this video, I show you everything you need to know when it comes to choosing a marking gauge as well as using one too. It seems like an obvious concept, but there's a lot more to it than first meets the eye.
If you want to know more about my mark gauges, have a look at my measuring and marking tools on kit.