If you are calculating the checksum on an airgapped computer and generating the address on it, why not simply create the seed/private key on it as well ?
Because there is far less trust involved in performing a single SHA256 hash than there is generating your entire 256 bits of entropy from a piece of software, unless you've written the software yourself or read the entire code, but 99.99% of people can't or won't do that.
Is there a specific reason to not gather the entropy from an electronic device? Or do you just like generating it from scratch ?
As Loyce says, provided I am using a fair coin (or even better, a variety of coins), there is zero chance that my output isn't truly random.