Eh it never really occurred to me. lol
I suppose it's not really fair we should see them!
I don't think fairness matters in this situation. Why wouldn't it be fair?
Though I do happen to think that the flying types makes the pokemon being able to be seen in the garden, impossible. Especially if you put a Zapdos in the Daycare Center. It could easily fly away and leave. Even though Nintendo wouldn't make it that way, it is common sense. It wouldn't make sense to have selectable pokemon in the garden either. Plus the water types such as a Wailmer, or a Finneon don't have water to swim in because there is no water for them outback of the Daycare Center. I have always wondered where they have the put the Water type pokemon though.
In R/S/E they had the same pokemon outback but yours wouldn't be. If they wanted it to be out, and you could walk around with it, they would have put it in Diamond, Pearl and/or Platinum. Of course, maybe they didn't think of this idea though.
It would only make sense to have a lava place for the fire types as well because if people put a pokemon like Heatran out, then it could catch the grass on fire. Again of course, Nintendo wouldn't make it like that but it is common sense. Therefore, I don't think they will ever make it this way unless they have a really good plan for it to work in all ways possible.