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Why is atmospheric pressure measured at sea level?
Because it's convenient.
Back in the old days, scientists didn't have a lot of fancy gear. As a result, if they wanted to measure things, they had to find units and ways of measuring things that were easy to reproduce in many different locations. As a result, we get the following units:
1 atm is the average at sea level. Sea level was chosen because everybody can agree about the altitude of the ocean.
Zero degrees Celsius was defined as the melting point of water because it's easy to find ice to standardize calculators. Likewise, it's pretty simple to boil water to find 100 degrees Celsius.
A foot is the length of some long-ago king's foot.
"Sticks" are used to describe explosive power, as in "sticks of dynamite."
The atmospheric pressure decreases as you rise above sea level, but it is always 1 atm at sea level.
You get the idea. If you're trying to figure out how to measure something, it's always easier to go with something you're familiar with, and that's how these things were determined.