My bet is they still use Plato for a couple of reasons:
1) They've always done it that way for hundreds of years, and
2) In their mind it might be a little easier to deal with than a bunch of ones and decimal points and zeroes. Just easier to write down and keep track of and so forth than a bunch of extra digits.
To each their own. I prefer specific gravity because for a 1.060 beer I know I can lop off the 1.0 and add a decimal point to get approximately a 6.0% ABV beer. But the pros will say, that's easy -- it's a 15 Plato beer, so just multiply by 4 and throw in the decimal point to get the same ABV potential. Personally I think not having to multiply by 4 makes life a lot easier. But whatever.