You don't have to buy them, you can borrow them. This opens up a certain type of attack where you promise to pay interest to get control of the stake. If you're

You don't have to buy them, you can borrow them. This opens up a certain type of attack where you promise to pay interest to get control of the stake. If you're running a stake pool, you can pay interest on top of the stake reward. It's possible to run a staking yield farm, where you pay interest in a different cryptocurrency that you can mint yourself. These kinds of off-network incentives can disrupt the reward system. It's even possible to incentivize a lot of people to collude in a double spend attack if the rewards can be distributed to the participants.