M.-A. Lemburg writes:
It is not uncommon for Asians and other non-Latin script users to use their own native script symbols for numbers.
Japanese don't, in computational or scientific work where float() would be used. Japanese numerals are used for dates and for certain felicitous ages (and even there so-called "Arabic" numerals are perfectly acceptable). Otherwise, it's all ASCII (although it might be "full-width" compatibility variants).
Please also remember that Python3 now allows Unicode names for identifiers for much the same reasons.
I don't think it's the same reason, not for Japanese, anyway.
I agree that Python should make it easy for the programmer to get numerical values of native numeric strings, but it's not at all clear to me that there is any point to having float() recognize them by default.