The easy answer is that the piano is not the "reference" instrument. If you look at a violin, the A major scale is generally the first scale you learn -- not C.

The piano has C natural in the middle of a full 88 keyboard, which is why it is a "natural" scale to pick (C major only uses the white keys, while A major would use three black keys).

So if you are learning another instrument, your "base" note will sometimes be entirely different.

Many "non-western" tradition also aren't tuned to 440 Hz, and even many _western_ traditions don't use 440 Hz for A, 415 Hz is also common.

[addendum] And why Do/Ut, Re, Mi? Because of a Gregorian chant where each sound sorta fell into a vocal range of monks ... then you adopt Do to whatever "sound" the instrument is closest, many years pass and deviations and standardizations ...

[addendum] I recommend starting out researching Guido d'Arezzo (and the Guidonian hand) a bit, that is where we get do/re/mi .. which was used as a teaching aid (which is maybe why Solfege as a term is often used in combination to sight reading or singing from sheet music). Almost no matter what language someone sings/voices Do/Re/Mi/... ... it falls naturally into a vocal range, irrespective of octave.

And then one could look into the hexachord ... and deep into a very deep well of confusion.