The quick answer to the thread title is, "The masses" (including me). The reasons are simple. Mac Photos integrates with the "world's most popular" photo platform, iPhone and iPad. While its editing tools do not have the sophistication of others, they are more than adequate for many purposes, and far more extensive than the masses ever use - do you have any idea how many people don't even crop? They're also more sophisticated than many photo enthusiasts are aware, but what does that matter if Photos can't match Photoshop?
Yes, the Photos database takes up extra space compared to simply saving masters to folders, although in my experience it is nothing near the 50% overhead the OP measured. I have a 483 GB Library, 409 GB of which is the Masters folder. That means overhead is 74 GB (15%). 64 GB of that are thumbnails and preview JPGs, which greatly speed display when I'm browsing through the library. Most of what remains is edit metadata, which Photos stores separately from the image. But whether the edit data is saved in separate files/folders or bundled into a PSD, it has to be stored.
In the end, any photo management software requires additional overhead. If you want a management tool rather than a manually-managed collection of folders, it's going to take additional overhead. In theory, the benefits derived from the expenditure of additional storage space will pay dividends in productivity.