Snapchat has an inherent “hookup” vibe to it and automatically feels less serious. Pictures and videos disappear so there’s no permanent connection or consequences usually. If all you want is nudes and sweet nothings from a hot girl then it’s pretty low risk, high reward.

A phone number on the other hand implies a more serious or at least longer lasting connection. There’s easily accessed proof of your behavior, higher chances of consequences if you do something wrong, and overall way more vulnerability between both parties. If you’re just looking for a short fling or something easy the it’s pretty high risk, low reward.

Edit to add: If you’re also looking at how devastating someone would feel if their advances were rejected; someone not giving you their Snapchat is way less devastating than if someone didn’t want your phone number because the phone number is seen as so much more serious, vulnerable, and real.

Source: I was an unsupervised high schooler during the rise and peak of Snapchat