Moogle, i don't think that's the reason, although i don't know it myself. Till the 993 series, in which the engines we're air cooled (or "oil cooled"), it used a lot of oil because it did the same job water does nowadays.
It isn't about wet sump, because both the GT3, GT2 and Turbo has dry sump lubrification and still use a lot of oil...
Why? I don't know either.