Why do microservices architectures typically give each service its own private database instead of sharing one database across all services?

Giving each service its own database preserves four benefits that a shared database quietly destroys: independent deployment (a team can change its schema without coordinating with every other team), independent scaling (only the databases under real load need to scale up), fault isolation (an outage in one service's database doesn't take down unrelated services), and technology freedom (each service can pick the storage engine that actually fits its data). A shared database forces every schema change, scaling decision, and outage to ripple across every service that touches it, even ones that had nothing to do with the original problem. The trade-off is that combined, cross-service data access — like showing one customer's full profile pulled from several services — stops being a single simple query and has to be handled deliberately instead.