There are mainly 2 reasons for this.
The first reason is that the school is newly established. The first inspection should be conducted by the Ofsted within the first 3 years from school opening.
The second reason is that, over time, many existing schools have been approved to voluntarily convert to academies. After conversion, even though the name of the school, the address, the headmaster and the staffs may remain unchanged, these schools would be legally seen as a newly established school using a new URN (Unique Reference Number).
At any chance that you spot a school without an Ofsted rating, we recommend you to take a look at our 3 steps guide.