Because there's a lot of stuff in the air besides oxygen. I'd prefer to have just oxygen in my starter.
It may be an unfounded concern, but I've always been worried about having an infected starter ruin a beer. If foil is good enough for a starter, why don't we just put foil over the top of our carboys?
Maybe it's just me wanting to play with my new toy (aeration system) as much as possible, but I don't see the point of taking a chance if I don't have to.
From mrmalty.com: "Q: Should I add oxygen to my starter?
Yes. You'll get far healthier yeast and far more yeast growth if the yeast have oxygen throughout the process. Adding oxygen at the beginning helps, but the most effective starters provide a continuous source of oxygen. Oxygen is critical to yeast growth. Not providing any oxygen to the yeast can have a long-term negative impact on yeast health. Yeast use oxygen to synthesize unsaturated fatty acids and sterols, which are critical to creating a healthy cell membrane and good cell growth. With oxygen present, yeast convert sugar to carbon dioxide and water and they grow rapidly. With no oxygen, yeast create alcohol, grow far more slowly, and reach a lower total mass of cells.
There are several ways to add oxygen: intermittent shaking, a stir plate, pure oxygen, or an air pump with a sterile filter.
A stir plate is perhaps the most effective method. When using a stir plate, don't plug up the starter vessel with an airlock. A sanitary piece of aluminum foil or a breathable foam stopper is all you need. Bacteria and wild yeast can't crawl and a loose fitting cover will allow for better gas exchange.
Shaking the starter as much as possible, every hour or two, makes a large difference in the amount of yeast growth and health. With enough attention and good air exchange, shaking is just about as effective as a stir plate.
Continuous air from a pump and sterile filter can be effective too. The major drawbacks are being able to control the flow of air to prevent excessive foaming and evaporation of the starter. Shaking is just as effective as intermittent aeration with a pump.
Continuous pure oxygen from a tank or oxygen generator is both expensive and unnecessary. "
I use foil on my starters and carboys all the time, let's gas go out and nothing to fall in. The only time I would feel uneasy about using foil is if I had a fruit fly problem, they unlike wild yeast/bacteria can crawl past your foil into your beer. But then again, they can also get into an airlock.