Think about a globe.

There are lines starting at "the north pole" that radiate out down to the "south pole". By nature, every single one of those lines is exactly the same length.

Latitude on the other hand, are perpindicular lines which, by nature, ring the entire globe. The further north and south they get from the equator, the shorter in length they are. Accordingly, to permit uniformity and relate them better, the degrees, minutes, and seconds relate to a top-down view of the globe and at what point on them they are. You can't use length since (as mentioned above) latitudinal lines are not equal in length.

Hope I made sense. If not, I'll let someone else tackle that