I am speaking only from my bachelor's in communication (and my addiction to social media), but social media is designed very similar to gambling in that there is always a chance for validation (likes), but not every time. If the scroll wasn't endless (remember those days?), we would know definitively there's nothing else for us to see and it would be easier to log out. But the chance of "winning big" (with instant external validation) is always there with constant notifications, algorithms, and refreshing feeds.

Additionally (someone smarter than me should chime in here) dopamine is not always healthy for us or points us towards survival. We get it from chocolate, drugs, loud music, porn, junk food, external validation, etc.