You can adjust this in Settings, there's sliders where you can adjust how many overall can leave a team each year, you can also adjust a slider for user and for computer teams as to the likelihood how often they'd transfer. I adjusted mine in the first offseason and noticed a significant difference...guys who I would've absolutely lost, now mostly stayed. Also saw fewer guys from other teams entering. Now I'm just trying to find a happy medium, so that it feels realistic without going overboard, but you can definitely adjust things