My question is why is there a different test for alkalinity in SW when pH shows if your acidic or alkaline?
I understand your confusion. The pH/alkalinity relationship is somewhat complex but they are not the same thing.
I suggest you read this article. The “How To” Guide to Reef Aquarium Chemistry for Beginners, Part 3: pH by Randy Holmes-Farley.
From the article....
"In short, all that many aquarists need to know is that pH is a measure of hydrogen ions’ (H+) concentration in solution..."
"Higher alkalinity implies more bicarbonate and carbonate in the water, and together these serve to buffer the water against pH changes (that is, they resist the change in pH as additional acids or bases are added). So the higher the alkalinity, the lower the diurnal pH swing. Also, the higher the pH, the more effective is the buffering provided by bicarbonate and carbonate in seawater (up to about pH 9), so the higher the average pH, the smaller the diurnal swing. Additional chemicals in the water also help to reduce the pH swing; borate, for example, buffers against pH changes."