Why is Daisy immortal and able to live nearly 700 years? It's implied that she cannot die until she makes her wish; but we've seen plenty of other characters die before making their wishes, so having an unused wish is no protection of any sort.
It's stated to be a result of the 'tontine' wish that gave her a second genie. While it was intended as a clever loophole to get all three band members into Fun Town by having them joint-own a genie, the implication is that part of the intent behind it was for the genie to cease to have any owners only through the spending of the wish - accidentally granting the final owner immortality until then.
Similarly, why is Betty effectively immortal? Yes, her mother's dying wish is for her to be a "survivor", but that doesn't mean "live for over 600 years". She also suggests that she will die after making her wish, but there's no reason for her not to die earlier.
The mom's wish was for her daughter to "survive." The genie probably took it upon itself to interpret the wish as such that Betty wouldn't die of old age. Keep in mind that intent is taken into account with wishes - and a mother's love is a powerful thing.
