"Who wants to be DF'ed and treated like crap when they could keep it between themselves and God?"
Many do. Those that get DFd are mainly those that get caught by other JWs doing "serious" sins. They are then reported to the elders if they don't go themselves and confess their sin. A few sinners are plagued by guilt and voluntarily go and confess their sins without being caught by anyone. But they are probably in the minority.
The number of people who are DF'd and reproved is not representative of all the "serious" sinning going on. Many JWs secretly carry on "serious" sins like smoking, celebrating holidays, fornication, etc. Those that are reproved or DF'd are only those that have been found out.
There is even a JW expression - "living a double life" - to refer to a JW who is a good JW in the presence of other JWs, but is breaking the rules when not around JWs.