Before someone asks why there isn’t insane inflation from banks printing an infinite amount of money for themselves, the Hong Kong dollar is pegged to the US dollar. In order to be allowed to print HKD, banks must have an equivalent amount of USD on deposit.
From living in HK, they all look very similar / have similar security features so most people are used to the different designs. Might be easier to scam tourists tho.
There was an infamous scam a while ago where Zimbabwean $1,000 notes were the same shade of orange as Hong Kong $1,000 notes.