Earlier quoted context omitted.
Because the internet at large is not going to do that 5 minutes of work - just like with web standards, it's up to browsers to support the decade-of-html that was produced before anyone gave a crap about the w3c.
Googlebot is a not a browser. It's a distinct "user" of your site with special needs. Think of Google as a retarded 30 year old user of site. If 67% of your users relied on the opinion of that single user, you'd damn sure do 5 mins worth of work to help him along. And if you didn't it would be your own fault for failing to get his reference (read: ranking).
Just like with browser vendors fixing bad HTML themselves in their rendering engines this is something Google has to fix themselves, because it's one fix for the whole internet instead of 11 billion fixes being delegated to people who just aren't going to do it.
It's a shitty job but they volunteered for it. And make billions doing it.
I am sure there are massive sections of the internet that are never going to get more than incidental traffic from Google - all the sites that just don't know/care about SEO, not to mention all the ones that just can't outdo the adsense and affiliate spammers and content farms.