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.

Hell, it took like 5 years for the market to get back to its peak after 2008.