Wow, you're right! It's crazy how you can go your whole life never noticing or thinking about something like this that is so clearly irregular, just because it's the way you've always seen it or thought about it.
I thought for a few minutes about the reason that units of currency are different and come before the number, determined there must be an obvious logical one. But, if there is, I can't think of it!
I think it’s so you can fit the cents symbol at the end e.g. $6.75c - but this isn’t really used anymore so the dollar sign at the front is a bit of a historical anomaly at this point.
Honest question, was it ever used in that combination? I've seen ¢ used for amounts less than a dollar, e.g.: 75¢. I (still) sometimes use it like that, because, I guess, it makes me feel adjusted to my environment, as an ESL :-) But I've never seen both $ and ¢ in the same amount/expression.
Ah, so I also don't think I have seen this in practice, but I suppose it's possible that it originated from pre-decimal-system currencies, like the old British money system, where perhaps it was more important to use separate units for clarity.
I suppose it's also possible it needs to sometimes be used (although, again, I have never seen this) in certain cases in more 'formal' situations - accounting, contracts and so on. I guess the format, with the symbols wrapping the numbers so as to make them easier to parse, does make sense.
I thought for a few minutes about the reason that units of currency are different and come before the number, determined there must be an obvious logical one. But, if there is, I can't think of it!