The gift tax exemption is co-mingled with the estate tax exemption, so you can just think of the gift tax as pre-death estate taxes.
As for why we have an estate tax, the general idea is that oligarcies and plutocracies are bad; that while you should be able to give your kids (and grandkids, and even great-grandkids) every leg up in the world, they should have to work and contribute to society; that the pernicious effects of wealth snowball as generations pass; etc. Add to that the fact that an estate tax has the least negative impact on the taxpayer (and their spouse), given that the taxpayer is dead, it's a pretty good tax. Especially now that the exemption is so high, I feel like the value proposition is pretty good.