?
Because relative velocities can be represented as vectors. And using vectors make relative velocity calculations simple.
E.g.
vAB is the velocity of A relative to B
vBC is the velocity of B relative to C
then the velocity of A relative to C is
vAC = vAB + vBC
(Note this is not simple addition of 2 numbers; for example vAB could be 20m/s north and vBC could be 5m/s southeast. The calculation must allow for directions as well as the speeds.)