There are plenty of quite profitable websites which do not have this requirement. It is almost peculiar to sites which are trying to show display advertising to groups of users larger than many nation-states.
You can make an awful lot of money with one commodity server if your business model supports it. I used to have an effective CPM of $80 and I know one which has in excess of $500. No, that is not a typo. (That is on six digits of pageviews per month.)
You know how much scaling you need when essentially get 50 cents a pageview? Not much at all.
FogCreek has, if I recall correctly, one database server. I haven't read how many total machines they're using recently but its a "count on your fingers and toes" number rather than a "zomg we need a colo facility to ourselves" number.
I figured that most sites who would be interested in such a database either fall into the retail (recommendation) or the community (social graph) category. Both operate mostly on volume and the last thing you want is a hard bottleneck just when you're at the verge of becoming successful.
But well, if your business model doesn't require scalability on the web-tier then yes, these concerns ofcourse don't apply.