Because on a particular three/four-decade-old physical terminal[0], Alt-$KEY was transmitted as "1B [sequence for $KEY]". Eg, Alt-X was "1B 58"[1]. The escape key happened to also transmit "1B" on its own. Consequently, "Alt-$KEY" and "Esc, $KEY" were indistinguishable unless you went out of your way to engineer a race condition. This has never really been fixed, despite intermittent attempts.
0: I think VT100, but don't quote me on that
1: or possibly "1B 78", depending on how it handled shift; I don't remember exactly.
0: I think VT100, but don't quote me on that
1: or possibly "1B 78", depending on how it handled shift; I don't remember exactly.