Why do you pay a gift tax when you receive a bonus from your employer?

It's not a "gift tax", because it's not really a "gift" legally speaking. You do pay income taxes on it, just as you would on any other income.

There is such a thing as a gift tax, but it's usually paid by the person giving the gift, not the person who receives it. Yes, this applies even though the gift is presumably coming out of money on which income tax has already been paid. The purpose of the gift tax is mainly to keep rich people from doing an end run around an estate tax. If you die and leave Stately Wayne Manor to your son Bruce, there's (historically) going to be an estate tax due; the reason for the gift tax is so that you can't just hand Bruce the keys minutes before you die and say "it was a gift, not an inheritance, so no tax for you, Mr. Uncle Sam."