I'm actually staring at the 3rd edition Campaign setting book right now so I don't need to double check anything. If you think SCAG has more information than it then you need to go back and triple check.
Let's do the math. The 3e book has 315 pages of content, pre-index.
8 pages of monsters. Useless for players.
10 pages of adventures. 14 pages of DMG content.
12 pages of history and timelines. Handy for DMing, needless for an adventurer.
Crunch. 24 pages of class options, most of which is generic feats and 13 prestige classes, half of which are generic.
22 pages of spells.
24 pages of gods and 4 pages of cosmology (vs 21 for SCAG. Not much different).
12 pages of organizations, with 3 1/2 pages being statblocks. So 9-ish. Compares with the 10 pages of backgrounds.
12 pages on races including a full pages of names, a page on powerful races. Compares with 17 pages in SCAG. And since less pagecount was going to racial stats for the genasi and tiefling, there's a LOT more lore.
6 pages on the classes and their role in the world. The SCAG gives each class more.
20 pages on life in the world. Okay, the Campaign setting is better here than the couple pages on these topics. Still, this is a lot of reading for stuff that's often in the DM's hands. Like trade.
The bulk of the book (132 pages) is on geography. 11 pages of which (almost 10%) are taken up my NPC statblocks. 17 pages on the Sword Coast & North region. Maybe 20 at most (including statblocks). This compares with 60 pages in SCAG. Which means SCAG gets into a
lot more detail on those places. Even if you include a chunk of the Western Heartlands and other areas, SCAG is still well ahead in pages.
Also, not to mention the fact that while the 3rd edition campaign setting gives you regional history, life and society, and important sites, it also gives you plots and rumours.
I'm not sure why plots, rumours, and adventuring sites would be necessary in a player book: that content is to give a DM ideas and material to work with. And only so much regional history is necessary.
So SCAG has more detail on classes. And more detail on races. It has comparable detail on factions and power players. And comparable detail on deities (and likely more detail on fewer deities). While there's less detail on the entire campaign setting, there is far more on the expanded Sword Coast region.
But this comes at the expense of life in the world details, which are often the purview of the DM anyway.
And while the book is short on DM content and details, that's not what the book is meant to include. You can't fault a player's book for not included DM content.
SCAG is not a perfect book and could have been larger and included more details, but you could say that about any setting book. There's
always something else to say or include. And it's unfair to say it's lacking information and detail.