I can't speak for all teachers, but Summer isn't purely vacation time.
True, I don't teach summer school. True, I take as many vacations as monetarily possible. True, I stay up late and sleep in late. True, I watch A LOT of Netflix!
However, I am also busy thinking, planning, reading, learning, experimenting, practicing, discussing, and working towards my goals for the next school year. My brain never stops. It's always wondering what I can do better next time, and so much of my Summer is spent pondering this.
I will be providing several training sessions for teachers prior to the first day with students. To prep for these, I have been watching videos to remind myself about best practice, what I already know (like strategies and activities), and add a few more things to my existing toolbelt.
Honestly, I am not different from your students. Give me a few weeks vacation, and my brain turns to sludge. Don't get me wrong--I firmly believe we all need to give our brains a break--but there's also plenty of days of summer that I feel I need to keep my brain lightly engaged so that I don't go completely off the rails!
The time I spend "thinking" about my work enhances the work I do later on. And let's be honest, summer provides us with the time we all so desperately need! Some carefully constructed planning throughout summer can lighten the stress of planning and prepping during the upcoming instructional year.