But still dont see any reason to restrict that mechanicaly
If you demand it ...
Please quote me any rule that strictly forbids any Wizard to be so faithfull and religious, so he manage to reach any deity that would be able to provide him such powers.
Please quote me a rule that enables any Wizard to cast Cleric spells (emphasis on "any", so not including multiclassing into Cleric or taking a specific feat). Wizards can be faithful and religious - sure - but that alone doesn't grant them access to Cleric spells.
The reason to restrict it mechanically is that mechanically Cleric spells are given to Clerics, not Wizards. The classes were designed with this lack of spell overlap to help balance them and make each class feel more distinct.
