What are you doing to actually keep them at your company? I left a company after they invested a lot in training me. They gave me very little raises and already paid poorly, no guaranteed bonus, bad vacation hours, and no opportunities for promotion. They were shocked when I left, even though I had asked for very modest raises and was way more productive than the "seniors" at the company.

Most companies outside of FAANGs treat their talented juniors like crap, so of course they'll leave.