They could but then you open the mobile app or another computer and the key just isn't there. They could generate one subkey for each device but then you risk user emails being impossible to decrypt if they ever lose that device. Hell I'm a programmer and I somehow managed to get my own master key deleted because I ran smack into some gpg bug which I then reported and sent a patch for. If I can't do this without deleting my keys and being forced to revoke them from keyservers immediately after publishing, what hope do end users have?
The most secure solution is to generate keys on an OpenPGP smartcard like an NFC enabled YubiKey and use that key everywhere. Even that's incompatible with maximum reliability: YubiKeys can and will eventually fail and when they do your keys are gone. So you can't generate the encryption subkey on the smartcard, you need to generate it on a secure device, back it up to paper just like the master key, and then copy it to the smartcard. Otherwise you risk being unable to decrypt data later.
It's an incredibly hard problem and it's full of tradeoffs. I can at least respect their attempt to solve the problem.
The most secure solution is to generate keys on an OpenPGP smartcard like an NFC enabled YubiKey and use that key everywhere. Even that's incompatible with maximum reliability: YubiKeys can and will eventually fail and when they do your keys are gone. So you can't generate the encryption subkey on the smartcard, you need to generate it on a secure device, back it up to paper just like the master key, and then copy it to the smartcard. Otherwise you risk being unable to decrypt data later.
It's an incredibly hard problem and it's full of tradeoffs. I can at least respect their attempt to solve the problem.