Because the C Major chord (or minor chord, or any other chord starting with that note as the root; there are several) is based on the C Major scale. C D E F G A B C. The first note is called the root or "tonic." Each note following has a name based on its position in the scale; 2nd, 3rd, 4th, 5th, 6th, 7th, and octave (the next same note, in this case C). So C1, D2, E3, F4, G5, A6, B7, C8 (octave). When you spell a simple major triad, you use the 1, 3, and 5 notes (or root, third, fifth). In the C scale, these are C-E-G. C is the root of the chord, the tonic of the scale, and it doesn't matter whether you change the order to E-G-C, or G-C-E (which are called "inversions"), the three notes still spell a C chord and the root is still C.
Why it sounds that way? Because of how music works, and the physics involved ("Because God says so, Davey"). Why do we _call_ them by those names? Because people spent several centuries working out the concepts and rules of Western music theory. To a degree, the names are arbitrary, you could call "C" "Gleepdorp," and a "scale" a "Kommundaluklanna-buldanna" ("A rose by any other name"). But those decisions have been made for us, and its probably best not to try and reinvent the wheel when so many people worldwide accept the rules of a language system (in this case, music) "as-is."
GJ