In the US its always been the standard as far as I'm aware. I think its so its impossible to confuse cents for the dollar amount if you have poor handwriting. For example mistaking the value of this check as $401 instead of $4.01

Every person who has ever written a check ever. I haven't written a check in many years though, so I can understand how you aren't familiar

It's common on the fully written line but I've never seen it on the numeric line. He was a weird one.

As an American, it would be weird for me to see it as a decimal, however I can't say I've ever really thought about it. It's just "traditional" for checks to be written that way here, and how we're taught in school.

exactly my reaction... as an european, that is strange way of writing 4.01.

…who the heck pays forty thousand bucks for a check

Who the heck pays for a worthless signature?