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.