[iamlolz]

Because a business shouldn't constrained to fit an available domain. Dropbox originally registered getdropbox.com because dropbox.com wasn't available. If they'd worried about an available domain name they wouldn't have ended up with something nearly as fitting.

[k__]

Also there are a bunch of top-level domains available and ever growing. So availability shouldn't be a big issue.

People should be more concerned about not choosing common words. Mozilla and Google can pull off Rust and Go, but you probably can't.