Because the idea of a slave has immediate connotations of people being enslaved. I'm not talking in some broad philosophical sense of rightness, I mean it brings to mind, for me, thoughts about human slavery that are uncomfortable.
So what? Are we going to find a new word for "driver" because it brings to mind being driven to do something, without autonomy, perhaps against your will too?
Are we going to remove "duplicate" because we can somehow imagine that genetically identical twins are offended by this?
Are we going to remove "admittance" (inverse of impedance) because people will be uncomfortable when reminded that they weren't admitted to their first choice institution?
These are all equally ridiculous to the master/slave debacle.
Those are all ridiculous examples because they are purely hypothetical, but you're missing the point; if enough people felt strongly about those then yeah, we'd probably change the way we talk.
