Probably because it is not stored properly. To prevent ice crystals from forming as it freezes, make sure it is well-sealed. I recommend covering the ENTIRE surface area of the mixture with plastic wrap (with no air in between). Also, there are fancy u003ca href=u0022https://amzn.to/3zpuBPnu0022 target=u0022_blanku0022 rel=u0022noreferrer noopener nofollowu0022u003eice cream containersu003c/au003e you could buy.