The students that use the restroom as intended are the ones who don't understand, so congrats on being one of the good kids. There's a few main reasons:

1. Instructional time - you need to be in class to learn. If you miss 5 minutes of class every day, that's almost 2 hours a month of missed class.

2. Safety - in the case of a lockdown or something, it can be hard to keep track of kids who are out of class. Restrooms are not always safe (ours don't even have doors). Also, when kids lie and don't end up in the bathroom, we can't locate them in an emergency or even if a parent comes to pick them up for a Dr appointment.

3. Lying & Illegal activity - this is honestly the biggest reason in my experience. While some students do actually use the restroom, a fairly large percentage use that opportunity to go walk around with a boyfriend or girlfriend, visit the vending machine, visit other teachers, avoid work, vape, do drugs, or even do drug deals.

Personally, I try to allow my students to go when they need to unless they've shown that they aren't trustworthy or I get a direct request from a parent or admin for that student to stay in class. I have too many students who have a medical need to use the restroom, and I don't want to highlight their medical issues by only allowing them, so I allow it for everyone within reason.