I'm on mobile so I can't link my sources properly, but I will note what they are.

Hess began making the toy trucks in 1964, and originally sold them only at their gas stations. The trucks were only released around Christmas to provide a cheap gift opportunity for parents, as the trucks only cost $1.29 and came with batteries. The company only made a limited number of trucks every year, and they sold out quickly (Source: Hess company website, Company section, History). I believe this would contribute to the collectibility of the toy trucks.

At some point, the company began giving the trucks to their employees that wanted them for their children as well. My father has worked for Hess for around 30 years, and I remember him bringing the trucks home for my brother when we were younger. According to him, they don't just give the trucks to employees anymore, but they do offer them to the employees for sale as well as selling them in the gas stations.

John Deere made toy tractors years before that, sold through their dealerships. I got several from my grandfather. Other heavy equipment manufacturers do the same, Caterpillar, Case, and Peterbilt, off the top of my head.

All of these are highly collectable because they were produced in small quantities.