For variety and also watering can upgrades. As of now, there's no time in the Winter to upgrade it without interrupting pet friendship gain as there is in the other seasons.
If you pet and water your pet every day, you reach maximum friendship in 55 days, so that can be done before winter. Petting also gives double the points of watering, so I would not delay the watering can for those 6 points.
But just because pets should not have to go out in a snowstorm, I would like to see your suggestion implemented.