If it had anything to do with checks, others woulda evolved convergently. Not the case.

Not only dollars are written that way in American English but most (not all) other currency symbols and abbreviations also, like £3.21 and ILS 3.21. The only other thing I can think of is AD 2021 (which is not a unit so doesn't count).

$ before but cents symbol after, for extra chaos. (I don't mean $0.99, but rather 99c (where that's the symbol, which I don't know how to add here).

I disbelieve the checks explanation. It's just as easy to add numbers at the end as at the beginning. I don't know how checks were written historically, but when writing checks by hand in the US in my lifetime, we don't write the symbol at all -- there's a place to write the number and another to write it in words.

Fascinating question. See https://english.stackexchange.com/questions/34013/why-is-the-unit-of-measure-placed-before-the-value-for-currencies-are-there-oth for some discussion.