Looking at the Ghoul and the Ghast from the MM, we see that they both have a bite and a claw attack. The claw attack has two points more for the attack modifier which seems to indicate that the proficiency bonus is not added to the bite attack.
Now, bite has slightly higher damage, but claw has an additional paralysis effect. Given the attack modifiers and the paralysis effect, it seems like the claw attack will almost always be superior, even against undead or paralysis immunity, since 2 points on the attack modifier are usually better than two points of damage.
Ghouls and Ghast do not have a multiattack using both attacks which might make it necessary to balance the total damage of that multiattack.
For these reasons I suspect, that this is based on lore rather than mechanical / balance reasons.
I am also pretty sure these two monsters are the only ones having an attack without proficiency bonus.
Is there published information giving a reason why Ghouls and Ghasts are not proficient with their bite? I am looking specifically for published lore to explain this.
Best Answer
One of the designers offered this commentary on the issue. While not an official ruling, it offers some clarity and is in the Sage Advice Compendium by WoTC.
This gives a good idea of the thinking behind the idea.
So, in terms of lore, it's because ghouls/ghasts are bad at biting.