the stock market goes up and down daily, but with a functioning economy it will overall be up. from a distance you are right, something that keeps going up keeps achieving new highs.

but, if you follow the market on a daily basis, you will be paying attention to the market swooning for periods of time, maybe with a small recovery and then another swoon. New "all time highs" do provide convenient notifications that the economy has now grown from before. Without new highs, your investments are treading water.