Companies have to use some metric to determine minimum qualifications. Some metrics are more difficult to apply than others, and companies want the easier ones. Thus, total hours is the result. It can be difficult to determine whether a 500-hour pilot has more experience than a 1000-hour pilot, that takes an in-depth interview. But limiting interviews to 1000-hour pilots makes the job more manageable. It's not perfect, but it's a usable metric for a start.