Because it removes I, L, O and U from the alphabet. Dropping I/L/O eliminates the characters people confuse with 1 and 0 when reading a code aloud or copying it from a screen, and dropping U avoids accidentally spelling offensive words. It is the right choice for anything a human will transcribe: licence keys, recovery codes, short IDs.
How long should an API key be?
At least 128 bits of entropy, which is 22 characters of Base62 or 32 hex characters. The entropy figure shown above the output tells you exactly where you are. Anything under 80 bits should be treated as guessable at scale.
Is a custom alphabet as secure?
The randomness is identical — it always comes from the browser's CSPRNG. But entropy depends on alphabet size, so a 10-character alphabet gives you 3.3 bits per character versus 5.95 for Base62. Duplicates in a custom set are removed so they cannot skew the distribution.