They don't have to .. i have 11 websites and one of them receive 4-7k unique visitors per day. so i hosted all my websites on a dedicated server.
I'd put them on shared hosting, provided they wouldn't overload the server with traffic. Just get a plan that offers unlimited addons domains so you can have as many websites as you like on one account.
I am thinking about that too, VPS should be listed on poll I have 14 site and hosted under my VPS (+ with a few hosting client in Bangladesh), thinking to take a dedi box soon.
i have a few websites and i keep them on shaired hosting to save a few bucks. when my traffic will increase a lot and cpu usages will be on high, i will take those to dedicated hosting!