The reason is in part because miscarriages are common. They can happen for no discernible reason so unless there’s signs of an underlying cause, there’s nothing to look into. If there is a sign on the first miscarriage, they will start tests.

My wife’s miscarriage was due to a sub-chorionic hematoma, and we knew from blood tests she had abnormally low progesterone during pregnancy, which can weaken the lining. it was also a twin pregnancy and those are more risky. So with her next pregnancy, the doctors monitored her a bit more and everything was normal, so we think the issue was the fact she was put on aggressive hormone blockers to treat her endometriosis before that first pregnancy, but had been off them for months before the second.