I think the issue is that rental and fleet companies typically buy the cheapest, most dreary, exceedingly average vehicles.
The cars don't have lower resale value because they're used as rental cars, they have lower resale value and are used as rental cars because they're generic, average, sensible cars in every sense.
There's nothing wrong with that, but typically cars that have strong resale value stand out in at least one way (reliability, entertainment, comfort). Most mass-market commodity cars drop like a stone after initial sale.