First of all, my personality. If you don't want to eat because you are introverted, my suggestion is to get together with your roommates as much as possible. If my roommates often invite you to dinner, you can choose a more suitable time to be with them instead of saying yes every time. After all, this is normal social interaction. If you refuse your roommate's invitation every time, after a long time, your roommate may habitually not invite you, and then you will feel isolated, but it's actually because you don't want to go, so if you don't particularly resist, try to go.
The second is your own reasonable planning. If it is because you have a reasonable plan for the day, then you can explain the situation to your roommate. You really can't go to dinner. Usually, they will understand you. But you can't schedule your time to be full every time. This is also a healthy way to have a proper rest. If you have your own food every time you eat, your roommate may miss you, too. Don't you want to be with them? This will lead to another contradiction, so if you can avoid this contradiction by getting together once in a while, try to get together once.
Finally, the relationship with your roommates. If you have a good relationship with your roommates, they invite you to go if possible. After all, your relationship is still very good, and dinner is also a way to enhance feelings. If you don't have a particularly good relationship with your roommate, you can refuse some of their requests appropriately, although it doesn't rule out that they want to have a good relationship with you. However, if you attend many roommates' parties and don't get along well, you will still be invited next time.
To sum up, let's go to the roommate's invitation or try to go. If you cannot leave for some reason, you can explain the situation to them.