Look at it this way. Two twins work their entire careers in the same job. Same pay same everything. One puts away money in an IRA, 401K and invests in the financial markets. The other buys a boat, a second home and vacations in Hawaii every year.
At retirement the individual who squandered his money gets SS tax free (he has little income other than SS). The frugal one meanwhile pays tax on his social security benefits.
Is that fair?