This has never been a problem for me. The job I stayed the longest was almost 4 years, most were 2 years or less. Switching jobs is good for your development and growth.

If you want a good answer for that question, tell them you switch jobs once it's become clear that there's not enough room for growth in a company anymore. And this growth isn't just about salary; it's also about learning new skills, new technologies, new responsibilities. If they want to keep you longer than that, they need to keep investing in you.