???
What bicycles are you referring to?
If you are thinking about bikes in WalMart or similar, mostly true,
though Wal Mart now has a "road bike."
Check the bikes in your local bicycle shop (LBS). You will find that
bikes have a great variety of tires.
The "road bike" generally has tires from around 700x19 up to about
700x32, depending on the bike - very "skinny." Touring bikes are
generally in the larger category of skinny tires.
Hybrid bikes will go from about 700x32 and larger - sort of medium
skinny.
Mtn bikes will be measured differently, with a 26" wheel, and will
indeed be "fat."
Same with "comfort" bikes.
Here is a link with a variety of tires. Go there and check out the
sizes
http://www.nashbar.com/subcategories.cfm?category=121
Here is a link to a variety of bicycles. Go there and check the
different types out, with the different types of tires.
http://www.performancebike.com/shop/category_bicycles.cfm