if you're big enough to afford TV advertising or enjoy unpaid media coverage, everyone's sure to know your name anyway.
I think they don't do it, because they don't realise the benefits. Any company, no matter how large, will want to rank for keyphrases / words that can gain new business. I think in particular traditional marketing agencies, which are the ones that big companies use, simply haven't a clue how to market on the internet. All the better for the up-and-coming.
The company i worked for was a web develpment company, not a marketing firm. It's not that we didn't suggest it to them... although come to think of it, google wasn't really in the picture at the time, and it was just as easy for them to spend the money to buy keywords to get ranked. Maybe now a days they are doing more of it, i'm not sure. i've since moved on to being a social worker