The employee master is not just an HR list — it is what the CRM assigns leads against, what Projects staffs a team from, and what payroll pays. Getting it right once is what stops four systems disagreeing about who works here.
The list that drifts. A separate directory per app is one that is wrong within a quarter, and the leavers are what nobody removes.
Join once, and every app already knows who you are and what you can see.