No reason at all - other than i find a protein shake satisfying and filling - much more than salads ect. Plus its much quicker and cheaper than other alternatives around me at work.
I could eat normal food at work and often do, there is nothing stopping me. I am trying to limit the calories I eat during the day as this is a period of 8 sedentary hours where I can easily over eat out of boredom/repetition - So I find it helpful to fast through the day at work and then train fasted straight after work before having a big evening meal.
Its tough feeling hungry when everyone else is eating cakes ect though! If I was there alone it wouldn't bother me.
This is the most arse about face approach available. If you're new to this, fuel your workouts or you're not going to get enough work done. Get the work done and you can eat more.
People come up with so many convoluted ideas. It's not hard. Eat things you can identify the parts of (thanks Lew) and work hard. The rest is important, but only when you're looking for every last inch.