At least to my palette, whatever I can buy is going to be tastier than what I can make. It's also fun to spend money on things and get out of the office.
For me, it's the exact reverse. :) I rarely buy a lunch that I couldn't have made better myself, for less money. But then again I also find cooking fun, whereas for others it's a time sink.
I love cooking. And sure I could, possibly, make a better burger or taco at home than the one I buy at the food truck, but I'm not going to be able to do so at work.
I wish I could cook at work!
There are plenty of meals that you can make in advance and reheat at work, but I see your point. Working remotely makes this a non-issue, though. :)