It is possible to identify candidates with low skills but high potential who can be trained. But most hiring managers are too lazy and incompetent to do this so instead they take the easy route of filtering based on education and work experience. Back in the early days of computing before it was really taught in most universities, IBM used to hire programming trainees based on the results of an aptitude exam. This worked pretty well and a similar approach could still be used today.
No, it's because as soon as they are trained they move on. And an increasing salary doesn't stop that.
People want to job hop to build up a diverse experience and no company wants to be the mug that pays an engineer during their bad years so that someone else gets to take advantage of it.
Mamy (most?) people would prefer to not have to job hop if the meager slary increases didn't make it almost mandatory. Even then, many people stay because they hate the idea of changing jobs so much.
