There could be a number of reasons for this:
1. Doctors are human beings with many different personalities. Sometimes, they get lazy and don't do the required drug background checks hoping that everything will be OK.
2. They may be busy and forget to check.
3. They may not do the checks relying on memory instead.
4. They may feel they are too busy to check.
5. They may feel it's up to their assistant to check.
6. They rely on the pharmacist to notice any contraindications.