[superprime]

At large enough size, commission/trading costs are not necessarily a significant factor--individuals are just not great at stock picking and timing the market.

[bunderbunder]

Absolutely true. However, individuals don't necessarily have enough money in their brokerage accounts to hit the point where trading costs become insignificant, especially if they're trying to diversify.

[HockeyPlayer]

At those larger size, the bid/offer spread becomes much more significant, and is usually a bigger cost than the commission.