I can answer the question in general. Because they're lazy. Americans especially don't have a clue about currencies and exchange rates and the like (not saying this is an exclusive American trait but they seem to be the most ignorant).

They buy something and they're told it costs €200. They have no clue so ask how much is that in real money or maybe they do say how much is that in American money. Today, of course, every last smart phone (even the old pre smartphone mobiles) has a calculator and even has a currency exchange key but they're too lazy to use that. Now picture going into the same shop and the clerk telling you that they can write the charge up in dollars if you wish and it is whatever in dollars. Maybe they ask which is better. Or the clerk starts throwing out the lies how you can lock in the exchange rate by doing it now or perhaps the clerk says nothing and presents you with a dcc'd sales draft to sign and if you start asking tells you it's a great rate and to compare it with the bank rates in the windows (of course we know the bank rates in the window are for cash and far far worse than the rate the cc companhy will give even if you're moronic enough to use a cc with the asinine should be illegal 3% ftf) or they may tell you the rate in your currency is shown for your convenience and is an approximation and you're really being charged in euro or it's done automatically by the terminal and they have no control over it (an advantage of course perhaps of chip and pin if you are asked for your pin before the fatefaul moment the exchange is done by the terminal) or sorry once it's done the transaction can not be undone (we've discussed that right here just in the last few posts). Or another is this will avoid the credit card foreign currency exchange fee of the naughty credit card company. I've even seen in some Caribean countries the lie that the government requires all purchases be converted to US dollars for Americans. Whether the cashier believes any of this is moot, those are the lies they're trained to tell.

So ultimately knowing how naive many travelers are, what do you think is going to happen? That's what keeps dcc going. (BTW you might find interesting some of the brochures for merchants outlining the advantages of dcc and of how their customers love to be quoted the price in a currency they understand!)