Because you haven't been to my house!?
The pros of towers are
1) Looks (most people like the look of towers over bookshelf speakers on stands).
2) Higher SPL (which doesn't matter if the bookshelf gets louder than you will play it).
3) Lower frequency response (which often doesn't matter because subs seem to like to be crossed at 80Hz or higher which a 6" bookshelf can handle reasonably well).
The cons of a tower are
1) Cost
2) If you sit close, the multi-drivers on towers spaced out with a greater distance apart may not have enough distance from the LP to integrate well.
3) Resonance - The larger panels of a tower are inherently more prone to resonances. More expensive towers incorporate computer analysis and are braced accordingly.
Wild card factor - not all towers and bookshelf speakers share the same design. For example the tower is likely a 3-way while the BS a 2-way. In this case, there can be real differences in how well the crossover is designed and the capability of the different drivers used. Which has better SQ all depends on the specific speakers!
My rule of thumb on this is if the savings allows you to step up to a higher level of performance by using a BS, do it! For example, the SVS Prime tower costs $500 each and the SVS Ultra BS costs $500 each. For equal cost, assuming you will use subwoofer(s),the Ultra BS is a much better option. The caveat is that if you had a truly humongous room, the BS may not fill it. However, if ultimate SQ is the objective the drivers in the Ultra are simply capable of more accuracy and detail!
I recently stumbled over Marshall Guthrie's comment on the Ultra Tower vs Ultra bookshelf which reinforces my thoughts:
https://www.audioholics.com/tower-speaker-reviews/svs-ultra-series-speakers-video
If cost is not a concern, and you are buying in a top-tier level of quality, there is not much penalty of getting towers (since cost is not a concern and you can't use the money saved to buy into a superior series).