The step-up in cost basis at death is primarily to avoid an accounting nightmare.
Mom and Dad (say) buy a house together in 1969. Dad dies in 1999. Due to unlimited marital deduction, no estate tax on half of house left to Mom. What is basis at this point?? Mom dies in 2019, say. You are the executor: you have to pay capital gains tax. Good luck funding out (a) what Mom and Dad paid for the house in 1969, and (b) what the "fair market value" of the house was in 1999.
Or Uncle Mookie (say) dabbled in stocks, with a Dividend ReInvestment Program. You are the executor of his estate. What is the cost basis in his stock portfolio?
Imagine you are the IRS auditor assigned to audit the estate tax return. Good luck!
At the very least, give everyone a step-up in cost basis at death to fair market value at date of death, lower the estate tax exemption, and tax most estates at capital gains tax rates (except the ones over the current level, keep that level and those rates as they are).