Because it costs a lot.  A lot, lot.  Facebook will make paper certificates available, and sites are getting ready to sell one at a time.  From CNN:

Buying through a one-share site is generally more expensive for customers than buying through a broker. Both GiveAShare.com and OneShare.com charge a $39 fee for their services, which include buying the share and procuring the paper stock certificate.

So a Facebook share worth $38 would cost you $77.  Plus:

Getting out is trickier than getting in. To unload, customers would need to deposit their share with a stockbroker and work through them. They would also have to surrender their paper certificate.

Apparently, most of the folks who buy these single shares, do them without the intention of selling. This is history  people.  This is art.  IGiveAShare is introducing a “new, improved” frame — “it’s larger and shows off the certificate better,” Roman says — specifically for Facebook shares. Customers can customize it with messages like “MARK ZUCKERBERG WORKS FOR ME! OFFICIAL FACEBOOK SHAREHOLDER.”