What do you tell prospective employers when they ask why you want to leave your current job?

I feel like there are no good answers to this. I’ve been trying to get away from my job since I started 20 years ago. Thing is, I never applied for this job. I never asked for this job. I worked for a family owned regional food distributor as a service technician. I worked on beverage equipment that we loaned to customers as long as they purchased the product for it. It was the best job ever. Most of the day was driving. I handled about 4 service calls a day. After about 7 months they sold the company and within 3 months we’d be out of a job. Then the owners who also owned a major manufacturing company said if you guys stay on till the end we’ll find jobs for you at their plant across the street. I had nothing but unemployment to fall back on so I stayed. I was offered a job in tech support because I had experience working on their equipment. I hated it. I’m not a cubicle person. I did everything I could to do work in the field and for many years that worked. I traveled periodically and saw most of the country over the years this way. Now there’s a new boss and all that has stopped. I haven’t been on the road in two years. When they ask why I want to leave my first thought is “I hate this fucking job and I always have. I never even wanted it in the first place. I didn’t ask for it, I didn’t even apply for it” but I know that’s the wrong answer.