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Why don't forced buy-ins of short sold stock happen much more frequently?
Many investors don't invest for the short term and so a stock "nose-diving" in the short run will not affect their long term strategy so they will simply hold on to it until it recovers. Additionally funds that track an index have to hold on to the constituents of that index no matter what happens to its value over the period (within trading limits). Both of these kinds of investors will be able to lend stock in a company out and not trigger a forced buy-in on a short term change. If the underlying long-term health of the company changes or it is removed from indices it is likely that this will change, however. Employee stock plans and other investors who are linked directly to the company or who have a vested interest in the company other than in a financial way will also be unwilling (or unable) to sell on a down turn in the company. They will similarly be able to lend their stock in the short term.