We show everything in today's dollars to prevent users from thinking they will receive more real benefits than will actually be the case.
If inflation is, say, 3 percent each year and one's actual benefit is $10,000 this year and $10,300 next year, because it rises by inflation, in real terms, i.e., in terms of its purchasing power, it's not any higher. The benefit has just kept up with inflation.
To report the benefit in nominal (actual) dollars, i.e., to show the benefit as $10,000 this year, $10,300 the next year, $10,609 the following year etc., might give the impression the benefit amounts will have more purchasing power through time, when, in fact, their purchasing power from their benefit doesn't change.