This is true, but it's also a good thing to go to lunch or have coffee with other people. You'll get personal enjoyment out of it, and if you are talking shop,

This is true, but it's also a good thing to go to lunch or have coffee with other people. You'll get personal enjoyment out of it, and if you are talking shop, the relationship you build could offset the savings of packing your own lunch. For example it's common that you will regularly have lunch with a work colleague, and then later they will recruit you to work at another company. Or they will give you insight on how to get support behind your project at your current company.