Not exactly sure, I think it’s just one of those things were we “people” have gotten so used to the design and how it’s supposed to look, that even if it’s not needed we keep doing it.

Since multiple companies manufacturer plugs and connections, some manufacturers may not need the holes as a part of the assembly process. Other may opt not to add them as they do not have a purpose (other then how we perceive a north america plug is supposed to look like).

Funny story, almost all standard north america three prong plug are installed upside down. The third “center prong” or “ground” is actually on the top per the original design.

Most new builds and electricians are opting to install this way now. The orientation has no impact on performance.

Older receptacles and even modern receptacles and cords do still make use of their holes.

It’s why if the holes are included there is specs they must follow.

This plug makes use of the holes for a solid mechanical connection that will only be removed when you pull on the collar to remove the pin that goes through the holes.