I work for a big corp who has always hired a lot of graduates for a grad program. We rotate people through two teams of 6 months each then place in a final team

I work for a big corp who has always hired a lot of graduates for a grad program. We rotate people through two teams of 6 months each then place in a final team in one of those areas or a third if necessary. Traditionally it has worked well as we get a good source of smart people. Recently though grads are leaving really quickly, like 18 months, even just a year. Maybe its because the job market is hot people can get mid level jobs with 1 year of experience. I don't think a recent grad with 1 year experience is worth paying mid-level salaries, but I guess that is all they care about. Training grads really sucks, they are negative to productivity, but I personally try to spend a lot of time interviewing, mentoring, training, doing extra code reviews because I've enjoyed it and want to help people. When people leave after 18 months with the company though its really disappointing, almost insulting. I feel like I'm wasting my time. Our team is going to stop asking for grads because we are really getting nothing out of it.