I had raised my thumbs a few times along the highway in Kampong Cham at desperate times when I couldn’t get a PassApp, or all PassApp drivers canceled my booking and the tuk tuk driver that I was familiar with was busy. Usually those who stopped asked me to pay so that’s not really hitchhiking.
I didn’t hitchhike to go from one city to another. I don’t think drivers, if they know what hitchhiking is, are willing to take hitchhikers. Considering transportation cost is not that high for the time being, you’d probably just choose to pay.