That’s a great question. The note has to retain a name, and I think since 4/4 is the most common time signature in western music, it’s universally called the quarter note regardless of time signature. I kind of think of measures like fluctuating markets…the dollar may be worth more or less depending on the day, but we still call it a dollar regardless.
To be clear, both the 3 and the 4 in 3/4 have a meaning. The 3 on top means that a measure is filled with 3 beats. The 4 on bottom tells you what kind of note gets that one beat (1/4 meaning quarter).
But like I said, great question