Why "bookshelf" sspeakers?

I just picked up a pair of Omega Super Alnico Monitors, a  rear ported single driver speaker for my bedroom.   Perfect with a 10" REL sub.    

There are no rules....  trial and error

You will pay a lot of money for that bottom octave done right with a pair of large floorstanders.  It is easier, and typically less noticed, for designers to cut corners with how the lowest frequencies are handled (i.e., driver size and quality, cabinet size and bracing, and design, e.g., bass reflex vs. acoustic suspension) to achieve a predetermined speaker price point.

As others have pointed out, “stand-mounted” is probably a more accurate description of most of today’s non-floor standing speakers.  If you can find a pair that righteously extends down to about 50Hz, or a little lower, then you can roll in a pair (or four) subs that have been purpose-built to handle the lowest two octaves with speed and power.  Other benefits of using subs include placement flexibility and relieving your main amplifier from the duty of driving the lowest frequencies.

For sure there is acoustic differences between bookshelf and Stand-mounted and Tower speakers ...

As there is difference between different speakers types too and not only about their sizes...

Acoustics taught us that there is a trade-off for each size and each types in relation to each room geometry and size and in relation to specific needs ...

Then debating what is the best does not means much ...

Acoustics taught us that we can use any types or speakers sizes optimally or not ...

The soundscape perception can be created by acoustic knowledge from any speakers...There is a trade off for sure between the acoustics factors in relation between types and size... But the good news is that we can create a relatively good High Fi soundfield with any speakers well embedded ...

My personal ultimate criteria is not size or types, more S.Q. levels/ price ratio...Because i dont have a big room now nor a unlimited budget ,...

But i dont mind, i use acoustics knowledge to shorten the distance between the best S.Q. possible and the optimal one for a price level ...

And it is not only enough to be acoustically happy, it is enough to smile at the astronomical prices paid by some compared to my own speakers value ... But for sure without modifications my active speakers are not good at all for me...Acoustics knowledge is way more than buying panels ...

We are all different in goal and tastes and needs... This is why i am very proud of my 150 bucks active speakers which modified gave me 50 hertz from their 4 inches woofer and a soundscape that made me smile with pride not envious at all but in the opposite almost taking pity of those who had invest way more for sometimes way less ...😁

FWIW I would never consider even a small HEA speaker a "bookshelf" speaker. A stand mount speaker is often more advantageous to the manufacturer than consumer. Why? Shipping/manufacturing costs/stands and even pushing subwoofer(s) are all additional profit makers. Is a Tower speaker more difficult with room placement? Probably, but most of the Audiophile community accept room acoustic treatments are a necessity thus eliminating most(all?) advantages that a stand mount speaker might possess.