Think of speakers like a flashlight which beams light in a forward direction, and very little light disperses off-axis of the flashlight. When you shine a flashlight at a wall, you generally get a tight spot of light with a small, dimmer circle of light around it. The same thing happens with the polar response of a speaker. The higher you go in frequency, the more the sound projects in a narrower and narrower beam. Eventually you reach a point where the beam of sound is very narrow, and the off-axis dispersion dramatically falls off. Like this:
Frequency is along the bottom scale from 50 Hz to 20 kHz
Angle is along the left hand scale from -200 degrees to 0 to +200 degrees
The scale on the right depicts sound intensity with 0, (unity), being orange, -3 being green, light blue being -6 and dark blue being -8.
At the left side of the graph, there is broad dispersion out to 200 degrees which goes out to 80 Hz, (low bass.) (This is where the statement "Bass is Omnidirectional" comes from. It's also why 80 Hz is the recommended crossover point for most speakers... because above 80 Hz, the sound becomes directional enough that the ear/brain can hear where it's coming from.) From 80 to about 500 Hz, the dispersion tapers and by 2 kHz, the sound has turned into a tight beam that continues to narrow more and more as the sound goes up in frequency.
Another way to look at it is to draw an imaginary line at 90 degrees from left to right until it intersects the dark blue color. By my eye, that is about 230 Hz. Anything above 230 Hz will be at least 8 dB lower than the original sound, when measured 90 degrees off axis.
Now, apply this to the above surround speaker positioning. Imagine placing a flashlight on top of the speaker, facing the way the speaker is facing. The "beam" of sound will be projected to the front wall, and the only direct sound that will disperse to the 90 degree off-axis listening position will be the low and mid-bass, below about 230 Hz. All the rest will shoot to the front wall, where it will be reflected back to the listening position. That reflected sound will be "heard" as originating from the front soundstage. IOW, the "surround sound" won't be "surrounding" you at all. It will be mixed into the front soundstage and draw the surround image to the front of the room, where it will do nothing but muddy up the front soundstage.
That may have been kind of OK back in the Dolby ProLogic days when the only thing in the surround channels was ambiance extracted from the front channels. However, nobody even puts Dolby ProLogic in modern receivers anymore. Its all digital, discreet surrounds, like Dolby Digital, TrueHD and Atmos. Discreet, directional sounds can be, and are increasingly mixed into the surrounds. With the speakers positioned as in the above picture, those discreet sounds would be virtually lost, or they would be heard from the wrong direction, (the front of the room.)
Fortunately, the photographer of that picture replaced those speakers and positioned them somewhat better, so the dispersion of the speaker was less off-axis of the listening position. That would allow the listener to hear direct sounds higher up in the frequency band. Still, the *best* placement would have been to use speaker mounts which allow the speakers to be turned 90 degrees and aimed down at the listening position. That would put the listener in the direct, on axis dispersion of the speakers and allow the best opportunity to hear the direct, discreet sounds at the levels they were recorded.
If you look at all the recent speaker placement diagrams from Dolby, THX, DTS, etc., they all show monopole surround speakers aimed directly at the listener's ears. Dolby has also gotten away from the 3" above ear level recommendation. Now they recommend placement just high enough to provide a direct path to all the listeners ears. That is the "best practice" for surround speaker placement.