Your logic is compelling, but you are "commingling," if you will two different tax systems.
(1) the income tax system (taxation of earned and other income
to one person, such as wages or a capital gain on sale of an asset).
(2) the gift & estate tax system ( a transfer tax system charged against gifts and estates -that final gift)
Capital gains are, as I mentioned above, simply a tax on the gain made on sale (to oversimplify a bit - sales price minus original investment).
Gift and estate transfer tax is charged on assets being transferred to another person without the expectation of anything in exchange.
Also, as I mentioned above, if you sell something to someone to someone at a price that's SO FAR below fair market value that the IRS deems it unreasonable (a price no reasonable seller would accept) they may recharacterize a part of that amount as a gift.