Perhaps people use the built in ones, because that's all they want to spend? They aren't the best, undoubtedly, but probably the keyboard sounds aren't necessarily the best either.... But they are still making music
Interesting to hear those who condemn 'studio elitism' in others be so derisive of those with lesser gear than their own.
BTW, if the preamp in your arranger's harmonizer isn't up to spec, some of them have line-ins. Use the preamp in your mixer (if better!) or an outboard pre to bring the mike cleanly up to line level, THEN input to the harmonizer... maybe use some decent outboard compression, too