In my field, I am lucky to get a 2% cost of living raise yearly, even with good to excellent performance reviews, but can get a 5-10% raise by switching jobs, even at the same title/level. I value other things, including stability, somewhat more than strictly maximizing salary at all costs, but it's pretty widely known that a good segment of employees will bail after 2-3 years for the primary purpose of getting a salary increase. Most people don't come right out and say "because you're stingy with the raises" at exit interviews, but it's generally understood.
Other reasons that I, or someone I personally know, have left an otherwise good job:
- Too much responsibility or too much stress
- Not enough responsibility or a feeling of stagnation
- Found an opening in a more desirable (for prestige or just personal preference) company
- Personality conflict with manager or team
- Wanted to work in the Big City (or wanted to avoid the big city)
- Wanted to obtain a specific benefit (like remote work or flex hours, short commute,
educational stipend, etc), or better benefits in general
- Wanted to change careers or industries
- Moving. Wanted or needed to live in a different area, for family reason or just preference
- Opportunity to work with someone they like/admire, a former coworker, friend or an expert in the field
Although these reasons vary in how 'good' they sound when leaving an employer, I think what you really wish to know may be better covered by question about What to say in an exit interview.
In general, exit interviews are very little to no benefit to the departing employee, and only marginally more to the company they are leaving. The usual guidance is to be positive, brief and vague. Focus on how much you enjoyed and/or learned from your current job, and that you got another offer you just couldn't refuse (no explanation of why is needed). If you like, you can name a manager or coworker who really made your time at the company pleasant or productive or educational, but you should avoid out-the-door complaints in most cases.
If you are leaving for a reason like outright abuse, ethical concerns, etc, you may wish to mention this on exit, but it's not a good idea to air minor grievances.