Why do all subclasses only use spells from the Player’s Handbook? [closed]

Some of the subclasses from classes like the sorcerer, cleric, warlock, paladin and druid gain addditional spells based on the subclass that they choose. These sometimes involve spells that aren’t usually part of the class’ spell list. However, when looking through the subclasses in Xanathar’s Guide to Everything, I’ve noticed that all the subclasses only gain additional spells from the Player’s Handbook. Once I noticed the pattern, I checked subclasses from other sources and noticed that they too only use spells that are in the Player’s Handbook.

Is this an intentional design? It seems like a waste to make 50-or-so new spells, to not use a single one in any of your subclasses. I could imagine you don’t want to cross-reference spells from multiple expansion books other than the PHB, but once you’ve got the book, you’ve got both the spells and the subclasses, right?