Why do Catholics often have a crucifix (with the Body attached) rather than a bare cross?

Because, though Jesus is now risen, he suffered and died for our sins.

We want to remind ourselves of that so we don't take Jesus' work for granted. It was the cost of our salvation.

Even after his resurrection, he showed in his body the 5 wounds of the Cross for his disciples to see.

With St. Paul, we continue that tradition today, portraying Christ and him crucified.