I think it's because the question is rather ambiguous - "convert the number to base-N" is a very common API, e.g. in C# you have Convert.ToString(long value, int base), in JavaScript you have Number.toString(base) etc. It seems that it just follows this pattern. If you were to ask me the same question, I'd probably do the same thing without any further context.

OTOH if you tell it to write a Base62 encoder in C#, it does consistently produce an API that can be called with byte arrays: https://g.co/gemini/share/6076f67abde2