Because the terminology hasn't changed even though the technology has.
Originally you'd put software onto an ROM. Then people invented PROM and EPROM (erasable programmable read-only-memory...already we've gotten confused terminology here, although it makes some sense, because you need a UV light to erase the thing, and generally you need a high voltage to program it), then an EEPROM (electrically erasable programmable read-only-memory...). And often the EEPROM wasn't rewritable without a special programming device being attached. Flash is a form of EEPROM, and now the programming circuitry is always part of the device.
So...yes, there's a contradiction in terms, but it's long-standing, and came about over time. And so, the image you flash to an embedded device is called a ROM, because a long time ago, you would have stored it in ROM. Language is like that. :)