Why are you using an old browser?

As someone new to web design and programming I am failing to see adequate support for the argument that we should write code that is supported by old browsers. I know that we can overcome the faults of old browsers with polyfill and web shim libs but like why are people using old browser versions in the first place? Why not just prompt them to upgrade their browser upon visiting the site? OR why not just NOT support old browsers and try to build a social stigma around using deprecated browsers?

I know that it's not wise to push away customers due to their browser preference but theres a reason why horse and buggy is not allowed on the freeway in the fast lane you know what I mean? I feel like if we keep supporting old browsers we enable the old browser users to continue their terrible choices.

I'm sure that there are some obvious reasons that I'm not seeing and this post is an attempt to lure them out to widen my understanding of the subject. Please comment with any case that you see as evidence for supporting old browsers.