What's a TSS Sensor? (Sorry, making a bit of a joke... the TSS is a collection of various systems under a fancy marketing name. What you're referring to is the MMWR Sensor that is used for the Radar Cruise & PCS systems)
The MMWR (Milimeter Wave Radar) Sensor uses a rather narrow beam to detect the vehicles in front of it and their distance. Looking at the Toyota Land Cruiser, Tundra, Sequoia, Tacoma, and 4Runner; my best guess is that it's mounted where it is because of some sort of regulation and/or engineering need.
In most of Toyota's vehicles the MMWR sensor is mounted behind a giant Toyota emblem; on the BOF vehicles it seems that it can either be mounted behind said emblem or just under it behind a special glass piece. Also looking at how it's mounted on the SUVs/Trucks, it looks like the times it is mounted behind an oversized emblem, that emblem sits about in the middle of the grill. The 4Runner's grill design sits the emblem more in the upper half and of course any of the PRO models have it mounted under the lettering because they can't hide it behind the PRO grill. So most likely due to the big cross bar in the grill being in the upper half, they decided that from an aesthetic standpoint it was just easier and cheaper to mount it below the emblem with a false glass piece like some of the other Toyota SUVs/Trucks.
I would say that halfway rule might be due to how narrow the beam is and because it also detects distance, if they mount it too high then the beam could theoretically overshoot something low like a sedan. Sure you could angle the sensor down more, but that might cause some blind spots/quirks. The MMWR is mainly designed to be mounted roughly parallel to the ground with maybe a slight downward angle. Think of it like mounting a laser or a red dot on your pistol/rifle, you can choose to either align it parallel to the bore or at an intersection point; if you mount it at an intersection point then before or after that point can be inaccurate, if you mount it parallel then all you have to account for is offset (which is roughly how the system is set up).
This actually brings up another point I should mention... Technically speaking, any time a customer deviates from the factory configuration; i.e. taller tires, a leveling kit, lift kit, new grill (even if that grill is say going from an SR5 to a TRD Pro grill, because the MMWR sensor may be a different part and/or mounted differently it counts as a non-factory modification) we're supposed to inform the customer that they can either have the MMWR Sensor (and thus PCS, Radar Cruise, and related functions) permanently disabled and/or the customer must sign a waiver that releases Toyota from any warranty and/or liability due to this system acting oddly because the customer has modified their vehicle. Now, whether or not that is being enforced I can't say; but that's Toyota's official policy on the matter since at least the K0B recall when they had to deal with lots of modified vehicles. Again this is likely to do with them not wanting to risk any liability if a customer plays around with their specifications on the angles and position of the MMWR Sensor assembly. It also could be due to some sort of federal requirement of how it's mounted? I never looked into what the DOT and NSHTA regulations are regarding these types of systems to be honest with you.
All in all I would say it's a combination of cost/design efficiency, the 4Runner's grill design, and possibly some sort of background regulation I am unaware of. This is more of a function over form design I think than anything, particularly since they're planning on replacing the 5th gen relatively soon they probably didn't want to do a 2nd facelift on the 4Runner (Toyota doesn't seem to do more than 1 face lift on a model within a generation in general, they probably hadn't considered installing a MMWR sensor when they did the original facelift on the 2015 models?)
Fun Fact-
The first generation Radar Cruise system was LIDAR based and those modules were mount as low in the front bumper (at an offset from center) as possible. In many cases they actually just cut up the bumper to fit in (i.e. 2nd Gen Sienna) if they couldn't fit it in the lower grill openings. Most likely this was done for a lot of the same reasons I suggest in the above post.
Post Note-
I had made this post days ago on my phone... seems the void swallowed it and it never got posted on the thread... FML