Given they are physically on opposite sides of the airport, how would you not separate them? The only option would be a tunnel on a 3rd level going between them. The weird and wacky design they came up with for the potential connection of them mentioned above show just how complex it would be to make that work (I think the preferred design has been posted here before - it's not pretty!)

Also keep in mind that the number of people actually wanting to go going between A and G airside has to be very close to zero. International arrivals (with the exception of a few pre-check locations, most/all of which probably didn't even exist when these terminals were built) all arrive landside, so it's maybe a few domestic arriving passengers, and some airport staff. (Not even airline staff for the most part, given a single airline generally wouldn't use both terminals). With the other terminals connected the number potentially goes up a bit (eg, going from T3 to Intl A makes sense for some connections) - but that wasn't relevant at the time as even Intl G wasn't connected to T3 until not that many years ago.