I’m not sure what you mean, both Photoshop and Lightroom use the Camera Raw. Lightroom is the Camera Raw plug in dressed up in pretty clothes but it has no functionality beyond what opening a photo in Camera Raw from Bridge or the Camera Raw filter in Photoshop.
Photoshop has access to ACR and you can use it in the filter menu on everything (any image loaded into the editing space, even after loading a JPG). But the native editing space of Photoshop is not just ACR. It is well beyond that, with layers and smart objects and all the rest. For instance, if you load a complex, multi-layer image into Photoshop and select the Camera Raw filter, it's only going to load the current layer (if you select more than one layer, you'll find that the Camera Raw filter is not available in the filter menu, because handling that document is beyond ACR).
So the native editing space in Photoshop is not ACR. You can open a JPG into the native PS space, without using ACR, as a simple raster-based image. Opening a RAW file will always require it, hence the R in ACR. And the C stands for camera for a reason, because PS can be used as an editing component of a work-flow that brings in pieces that a NOT RAW files from camera (vector based objects, etc.).
Lightroom doesn't have that kind of editing space and is
basically using ACR and bridge in respect to metadata editing, but it has far more than just 'pretty clothes' wrapped around it. To say it has no functionality beyond that is ignoring the cataloging (obviously, as has already been discussed in this thread) and everything you can do with that, plus the MAP module (one example of utilizing metadata in a way that PS is not capable of without some plug-in) and publishing / web-gallery abilities, plus the most important aspect: it's all integrated into a single user interface for all common users.
That last aspect cannot be overstated. Your familiarity with the core components from the pre-lightroom days is not useful to the current-day Lightroom user. If LR went away, they would not be happy going out and finding all the pieces that will allow them to do the core functionality, figuring out how it all works together, then learn how to run them independently — only to find out that even if they did, they would NOT have everything that their LR package did.
Then there are the 3rd party apps such as LRTimelapse that wouldn't work with just Photoshop.
Ask yourself why Adobe even made Lightroom if all the components were already there and easy to use. And be careful not to denigrate the millions of people using the application when you answer.
My answer would be: streamlined workflow. And that's valuable to a lot of people.
Put another way: if all you wanted to do with Photoshop were the Lightroom-centric editing capabilities (only those things within ACR, not layers / masks and all you can do with them), then Photoshop is an overkill and Lightroom makes more sense to more users because of its more sensible "image development" workflow with large catalog asset management.
Chris