“Spend other people’s money” is a really weird way to put it IMO - people chose to invest in the company because they thought it was a good investment. Investors would rather have the stock than the cash they paid for it. When the company does a buyback, the shareholder investment in the company becomes more concentrated. Each share represents more stock and less cash, making it possible to construct a higher-equity lower-cash portfolio. This more purely fulfills the investor’s revealed preference of owning the stock. If they change their mind, they can always sell.