Paul is responding to post-death baptismal rituals that the Corinthians have been practicing (or perhaps other people in the area of Corinth since he says “…why then are they…” with ‘they’ meaning someone else other than the Corinthians, ie. pagans in the area).  Since Paul isn’t clear in his description, precisely what the Corinthians or their neighbors were doing has been debated with many competing theories.  I don’t know if there is consensus on exactly what was taking place.  What is clear is that many societies had death rituals and some scholars believe the area near Corinth developed some that were elaborate (including pagan baptismal rituals) so perhaps this has influenced the early Corinthian church.  Hence, my parenthetical comment earlier.  Note that outside of the Mormon church this verse is understood to be misinterpreted by them since they use it as their directive for baptizing the dead.  If in fact Paul is actually referring to practices that ‘THEY’ do then obviously this isn’t something the Corinthians themselves were actually doing.  Note that a lot of chapter 12 deals with the resurrection of the dead so verse 29 in this overall context needs to be understood.