Why do chemotherapy regimens consist of multiple drugs instead of just one?

Sometimes not. Your oncologist will search out the most likely single or combination program, that you can tolerate to kill the cancer with out killing you... To put it bluntly. This takes a great deal of experience and trials with close follow up of your response to manage.
Chemo. Not all the time, we give chemo as multiple drugs combination. It will depend on the goal of the therapy- whether that is given for curative intent, or as a pallitive therapy - whether you can tolerate the chemo, the type of cancer , the stage etc. In the right setting we also give chemo as a single agent. When we combine chemo- we're aiming for a higher response rate and less resistancy to chemo.