The short answer is "because it doesn't need to be".
The long answer is to organise a demonstration on the terrain of your choice.
The medium answer is the subject of this post, and it starts with thinking about what you need in an offroad vehicle, irrespective of the driven wheels. It's not a long list:
1. Clearance - only the tyres should touch the ground
2. Power - the engine must be able to turn the wheels
3. Traction - the tyres need to be able to grip the ground
If you have all those three sorted then you'll keep moving, simple as that. So let's look at the Tomcar's design and see how it stacks up.
1. On the clearance front you've got a full 40cm under the body. That's equivalent to a Nissan Patrol...if it was running 40" tyres. The reason the much smaller Tomcar can manage such huge clearance is simple - fully independent suspension, and the underbody is completely smooth, no transfer cases, diffs or propshafts hanging down ready to snag. So even if the Tomcar does touch, it slides smoothly and it has built-in rock sliders.
Then you've got approach and departure angles of eighty degrees, something you won't find on a 4WD unless it's a competition truck. Ramp angle is also excellent too, you'd need a decent lift in a 4WD to get close. Really, just take a look at the Tomcar and you'll see that clearance is no problem.
2. Next up is power. The Tomcar doesn't have a hugely powerful V8, but with unladen weights starting at 800kg (about 1/3 of a 4WD before you add winch, bars and the usual gear) then it doesn't need a lot of grunt. It also has a Continuous Variable Transmission (CVT) automatic drive, which allows the engine to remain in the best rev range for either power, torque or efficiency while the belt-drive instantly and continuously varies the effective gearing to suit the speed. But that aside, the TM2's power to weight ratio is better than that of a 3.2 diesel dual cab Ford Ranger PX. There's also a low mode in the CVT transmission. Bottom line - the Tomcar won't be running out of grunt any time soon.
3. This just leaves us with traction, the ability of the tyres to grip the terrain. What we want here firstly is the driving wheels in good solid contact with the ground. The Tomcar obliges with rather decent 330mm of suspension travel at the front, and 360mm at the rear. There's also a cross-axle differential lock should even that suspension flex prove too little. It helps that around two-thirds of the Tomcar's weight is over the rear axle, which means that the drive wheels have good traction, and there's not much vehicle weight to shift around either. Indeed, in tests, the Tomcar has ascended slippery hills that that Land Rover Defender on mud tyres had to winch. The Tomcar's rear drive system also aids traction - due to the swing arm final drives, when you accelerate, the wheels are pulled down and under the vehicle, giving you traction almost all the time.
There's a couple of other Tomcar advantages. First is maneuverability. The Tomcar is small and agile, with excellent visibility, so the driver can take lines through rough terrain other vehicles can't even contemplate. Second is speed. Because the Tomcar is tough, small and light, with suspension not dissimilar to a desert racer then you can safely travel at quicker speeds than a 4WD. And momentum works for you offroad, allowing the Tomcar to magic-carpet its way over terrain other vehicles would be crawling over. You can even jump a Tomcar, and that won't affect its warranty (well, within reason, we said "jump" not "fly"). Try jumping your 4WD and see how it fares after it lands. And we don't say Tomcars are unbreakable, but we do say there's less panels to dent, sidesteps to bend and in general the Tomcar is built tough from the factory, not made so with aftermarket gear.
Then there's the side angle capability. The Tomcar is nearly as wide as a 4WD, so it can use the same ruts, but has a much, much lower centre of gravity, more like a Porsche than a Land Cruiser. This gives the Tomcar an unparalleled ability to spider its way around obstacles. And that's the thing with Tomcars - they are rough terrain vehicles, but they're not 4WDs so you can't use the same techniques, any more than you'd ride a dirt bike like a 4WD or even drive a Suzuki Jimny in the same way as a 200 Series Cruiser. But all three can negotiate the same terrain, and so too can the Tomcar.
Now you may be thinking that yes, all true, but the Tomcar would be even better with 4WD. And all else being equal, four wheels driving are indeed better than two. But back in the real world, all else is never equal and certainly not in the case of off road vehicles. 4WD adds weight and complexity, which starts to erode the core Tomcar advantages. Then where do you stop...you may as well say your 4WD would be better with twin diff locks, then tyres another 2" taller, then a more powerful engine.
The answer is you stop when you've got enough off road capability for your needs, and we believe the Tomcar is more than good enough for its intended use. But there's only so much we can write on the subject, and if you're not convinced then watch the videos, or come to us for a practical demonstration where you drive and compare the Tomcar against the 4WD of your choice. It'll be the long answer, but it'll be the final answer.
The truth is, the Tomcar's performance is beyond words: Before you drive one no explanation is possible, but once you do, no explanation is necessary.