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There are lots of “golden ratio” cabinet designs that are shallower and wider / taller.
Cabinets are usually deeper to avoid internal reflections from the back panel. If they’re active that’s where the amp goes too.
dbr-
Sure...
But here are in-wall speakers, the KEF Architectural ones and it's an 8" (20cm) driver https://uk.kef.com/products/ci200rs which only requires 4.7"/12cm of depth in a wall.
It is clearly possible to produce high quality speakers with shallow requirements on the cabinet, it only appears to be a choice to not do so.
Even the PSI Audio's on my desk have 4" (10cm) drivers and only have a depth of 6.7" (17cm) on the cabinet.
It's hard to even find great passive speakers which are shallow enough for a bookshelf and yet could fill a room with expansive and high quality music.
Velocio
On speaker size.. true the drivers have a certain depth. But I don't see why a cabinet needs to be in portrait mode, or that the cabinet needs to be 2x or 3x the depth of the driver.
An octagon cabinet (or a big square one) holding a driver with central tweeter would provide the cabinet more space and more consistently shaped space internally.
I'm frustrated in my shopping as everything that claims to be a bookshelf speaker option is above 30cm depth when I have height and width to spare.
I can only conclude the bookshelves suited for bookshelf speakers only hold art books.