Oooooh I see now, you took what I said and changed one of the words. Totally missed that before wareya's comment.
robots.txt and favicon.ico share nothing in common. Browsers do not automatically request robots.txt every time you load a site. If you count a bot as one visitor, they make up maybe 0.01% of your traffic. It's also an explicit request, the bot actually wants that file. Users don't actually explicitly ask for favicon.ico, their browsers do it for them when they type in pagename.html. It's also not part of the page like the icon is. favicon.ico is like having javascript.js and style.css in your root directory auto-scanned as well.
Unlike favicon.ico, you couldn't just slap a line in your HTML here instead, it would already be too late for crawling. If we tried to put this into the headers for a bot's HEAD request, then people with only file drop access to hosting wouldn't be able to change the robot settings.
That said, I'm always open to considering better ways of doing things. Maybe we could have web servers optionally parse robots.txt locally, and bots could get the information from a special OPTIONS / HEAD request instead.