Bitcoin's high volatility means that it's impossible to predict exactly how much value one Bitcoin will have tomorrow, next month, next year, or next decade. USD has comparatively low volatility. It's possible, but very unlikely, for USD to go to zero tomorrow, and while the uncertainty around exactly how likely USD is to collapse increases as you look further towards the future, most people would consider it less likely for USD to collapse than Bitcoin. Since markets are made up of "most people", most people are more willing to accept USD to exchange value than they are Bitcoin. Therefore, USD is a better store of value than Bitcoin