Why doesn't Obama quit?

Everyone is asking why Hillary Clinton is still running, when the real question is, "Why is Barack Obama still running?"  As Democratics who were sentient in 2000 recall, the president is elected by the electoral college, and the electoral college is selected by each state on a winner-take-all basis.  The first candidate to get 270 electoral votes (or 5 votes on the Supreme Court) wins. 

If, instead of delegates to the Democratic convention, you awarded hypothetical electoral votes to the Democratic candidate (Hillary or Obama) who won the primary or caucus in each state that has had a primary or caucus so far, and you awarded those votes on a winner-take-all basis, then the count as of May 8, 2008, would be as follows:

Hillary: 251

Obama: 208

(In this count, Hillary gets Texas because she won the primary, although Obama won the caucuses.  Nobody gets Michigan and Florida.)

Why does Obama continue to pursue his obviously hopeless quest for the Presidency?