Given that mundane gear typically has minimal mechanical modifiers, how is supplying a consistent baseline for such items to the game's core somehow worse than insisting that DMs "wing it" to fill in those gaps?
Personally, I'd much rather have the former than the latter.
Because, IMO, there simply aren't enough DM's who care about this sort of thing to justify an entire system for it. If you want it for your campaign, go for it, but don't pollute everyone else's campaign with it.
You're right. Sure, mundane items are just a few bonuses. But, that's where it all starts. You get +1 to Sneak for your Boots Low, Soft, and another +1 for the Face Paint, Black. Suddenly, you're Sneaking at 4 levels higher than what is expected for your level.
One of the goals in 4e was to limit the number of sources of bonuses, something they've managed to do fairly well IMO. You don't get bonuses to skills all that easily outside of some very specific abilities and items.
And all that besides, if all you want is a minor bonus due to equipment, the stock circumstance bonuses in the DMG 1 will cover everything. So, again, the rules guidelines are already there and all you need are the flavour elements.
If all you want is a big book of flavour, fine. Cool. Easily found in any library
Note, as I said above, things like strongholds, ships and whatnot are a separate matter. To me, those should be siloed off entirely from the rest of the game and turned into their own mini-game similar to how Affiliations were done in 3e.