First, that usually you would need to overpay for a player that is restricted. Secondly, that if an offer sheet is signed, the financial resources that would be used for that deal can no longer be used to go after other players for a week, during which time the team that holds the QO can make a decision on wether to match the offer or not. So during that week, they might miss out on other opportunities, while at the same time missing out on the player they signed for an offer sheet if the other team does match.
As for David Lee, there are a limited of teams capable of offering what he's looking for. And since offer sheets have to be for at least two years guaranteed, it makes it a bit hard.