Because eberron is strictly a better setting than forgotten realms.
or dark sun, or nentir vale, or planescape, or spelljammer, or greyhawk.... yeah ok im going off on a tangent here.
Anyway, i dont expect eberron stuff in BG3, i expect everything from the Players Manual and mabye some choice other things, Eberron stuff does exist in FR, for the most part, mostly due to 5E pushing FR even more into kitchen sink territory than it already was, but usually nowhere near the swordcoast and not very prominently
FR is the greatest D&D setting. And sticking stuff from other setting into FR started long before 5e. 3e adding Tieflings, Aasimar, Goliath, Raptorans, and Genasi from Planescape, 4e added Dragonborn, Infernal Style Tieflings, Dave's, 4e style Shadar Kai.
And hate the term kitchen sink setting, I prefer Immigration Evovling Setting. There are really good reasons why the Forgotten Realms are the most popular beloved and successful setting in D&D.