Because if you use specialized "technical" terms derived to serve special interest groups (i.e. medical profession terms, or computer geek terms) it presents the illusion that you belong to that special interest group. It is a way for people to belong and everyone wants to belong to something greater than themselves.
Learning the technical language of a hobby is a way to integrate into the social aspects of a hobby.
Well, Near as I remember, the term was introduced by some OVLR club members into the LR mail list in the 1990's. I think it was done basically as a put on to convince a know it all newbie that 'breakfast' was the LR hobby in term for the radiator bulkhead.
As an explanation they said the Australian Land Rover veterans use the grille off a pre Series III truck as a grill over a camp fire to cook their breakfast. THEREFORE because they used the grille to cook breakfast, the radiator bulkhead that the grille was attached to as called a 'breakfast' and the grille was still called a grille. Really great logic.
At the time several people pointed out that the zinc plating on the grille was toxic. That if the zinc plating was burned off they would be left with a rusty grille on their truck. And one or two asked that if the grille was used to grill breakfast why wasn't it used to grill dinner and why not call it the dinner?
No one seemed to ask the question "If the grille was used to cook breakfast, why call the radiator bulkhead a breakfast and not the grille?"
Somehow the OVLR club folks not only convinced their target person the the radiator bulkhead was called a breakfast because Australians used it to cook their bacon, but they also convinced a lot of relative newbies on the list as well.
The folks who started this had the Ottawa Valley Land Rover club web site and the Land Rover FAQ web site. So they added the term to both sites.
The LR email list is composed predominately of North Americans, the Two web sites are North America based, so since then everyone in North America who wanted to be a member of the Series Land Rover IN GROUP started calling the radiator bulkhead a breakfast because supposedly a unknown size group of Australians couldn't be bothered to carry a grill in their truck and removed the grille off their trucks, burned off the galvanized coating and used the grille as a grill only to cook breakfast and not lunch or dinner.
I thought the story was awfully silly at the time and that no one would take it seriously. But if it is in writing in respected web sites it must be true.
Since then somehow, a part that did not get removed from the truck became known by North Americans as a breakfast because supposedly a part that was attached to the radiator bulkhead was removed and used as a grill for cooking breakfast but not lunch or dinner.
Everyone wants to belong to something greater than themselves and using the group technical term helps them belong. But I still chuckle at people who use the term "breakfast" for the radiator bulkhead and it tends to confuse the Brits, Australians and South African LR folks who had never heard of the terms before the North Americans started using it.
When in doubt about the correct term of a Series Land Rover part look to see what the factory Land Rover workshop manual or owners manual, or parts book calls it. You can never go wrong by using the same term that the Land Rover factory does.