I have looked at many grandmaster games where White starts off with 1.c4, which later mostly transposes into a King's Indian. However, Black could have immediately played 1...e5, which seems slightly better. Why would Black not take this opportunity?
It is a matter of choice
Many grandmasters (e.g. world champion Viswanathan Anand) have consistently played 1.c4 e5 with Black. Others favour the KID or other defenses : Hedgehog (with b6), symmetrical English (1.c4 c5), Pseudo-Nimzo (with Nf6 and e6), Pseudo-Grünfeld (with Nf6 and d5), Réti (with e6 and d5) or Slav-like (with c6 and d5) defenses...
There is no definitive hierarchy between all those schemes. You may well prefer 1...e5 and you won't be the only one (this is actually my favorite retort nowadays) but there is no objective reason to think that this choice should be "slightly better" than other reasonable defenses. What suits us doesn't suit everybody.
The case with the KID
The King's Indian Defense has many pluses and minuses: lack of space, ressources for counterplay, long theoretical lines... But one thing that makes it attractive for many players with Black is that the KID is an holistic opening solution for everything but 1.e4 : if you wanna play the KID, you can go for it whether I start with 1.d4, 1.c4, 1.Nf3, even 1.g3 or 1.b3, and there is little I can do to stop you.
As a consequence players who are confident in their KID repertoire against 1.d4 have little incentive to switch to different schemes to meet 1.c4, and prefer to transpose with 1...Nf6 and 2...g6 rather than work on the theory of the pure English 1.c4 e5.