Question

Why height is measured from sea level and what is sea level?

Answer 1

The real answer is that when mankind was first dealing with real problems in physics, they had to choose a standard reference level on the Earth and at that time the one height they knew to be the same across the globe was the oceans. They knew that water settles to a level surface as a liquid and so sea level on a South American coast should be the same sea level in the Indian ocean. So, sea level was a convenient choice for a reference altitude.

At that time they did not know enough about the shape of the Earth, not being a true sphere. And they had no way to measure the relative altitude of the sea at different locations, so this was a reasonable assumption as a global standard altitude.