Probably because “tabletop” has such prominent T sounds when spoken aloud. “Handbook” is one word, but a huge number of people (me included for a long time) abbreviated “Player’s Handbook” as PHB.
Internally at Wizards we refer to the “TRPG” team, for what that’s worth. Man shrugging Why is it "TTRPG" and not just "TRPG"?
Like, "tabletop" is a single word.
— Sangjun… or is it? (@heavenspider) May 18, 2020
And to be completely fair, I still say PHB often, but I'm breaking the habit and saying PH more and more.
— Dan Dillon (@Dan_Dillon_1) May 18, 2020
Simple.
PH is acidity
TRPG stands for Tactical Role playing Game.
PHB is far less known term in science than PH is for the layman, as well.