There are three aspects to it:
- Rated max pressure doesn't necessarily have much to do with how much pressure the tyre can really withstand safely. All ratings on products are at least a bit below what the product can actually handle, but how big the safety factor is can vary a lot, for various reasons.
- Wider tyres do have an inherently harder job containing high pressure. This is essentially the point Criggie made in a comment, though I find it requires some more explanation.
- Of course intended use case, what L.Dutch's answer focuses on, also has something to do with it. MTB tyres are tough, but tough in different ways from road tyres since both are designed with different challenges in mind. However, this point is often exaggerated.
Let's first address the "inherent physics" point.
See, a tyre doesn't really "withstand the pressure" in the way e.g. an object in a hydraulic press withstands the pressure. In that case, the forces are directed on a solid surface, which distributes them to the bulk material underneath.
A tyre actually does something more demanding: it contains the pressurized air purely through surface tension. That effect is reliant on a curvature radius. The whole reason a tyre is able to withstand pressure at all is that the surface is curved, so that the purely tensile stress in the plies causes an inwards-pointing normal force against the air inside. The stronger the curvature, the less force in the plies is needed to contain the same amount of pressure. That's why even flimsy road tyres can withstand a lot of pressure.
So MTB tyres would already need to be stronger to withstand the same pressures. Now, they are stronger than road tyres in some senses – but mostly in the rubber part, the plies aren't much different. And the rubber is actually little more than a by-stander when it comes to the tensile stresses: rubber is just too flexible (which is why an inner tube by itself can hardly with stand any pressure at all).
However, all of this would still be somewhat irrelevant if the tyres were tubular, because ply rupture isn't the typical failure mode with over-inflated MTB tyres. But the tyres are clinchers, and the typical failure mode is the tyre bead slipping off the rim. And here we have an additional combination of disadvantages:
- Though MTB tyres are much wider than road tyres, they don't run on proportionally wider rims. Accordingly, the ply forces are not only stronger (due to the weaker curvature) but they also point in the direction most prone to pulling the bead off.
- The standard design of MTB tyre nowadays has a foldable polymer bead, which is more stretchable than the traditional steel beads, which also makes it easier to rip off.
- Tubeless means that the pressure can fully "follow" the bead and push it over the edge.
So all of this means that a typical MTB tyre will indeed pop off the rim if you try to inflate it to 90 psi, and certainly none could handle 130 psi.
But 45 psi, really that low?
No. Actually, even such a tyre can typically be run at significantly higher pressure, especially when using inner tubes. The question becomes, why would you?
Well, you've asked about one reason: road riding. And yeah, that is a valid reason, don't let anybody tell you that you shouldn't ride an MTB on the road. (In particular not someone who drives their MTB to the trails in a car, talk about efficiency...)
And indeed using a higher-than-normal pressure helps reducing the inefficiency of the wider tyres, going to the 45 psi limit certainly makes sense. At that pressure the tyres of course won't have as much grip and small-bump damping off-road as with a more typical 20-30 psi, but that's not a concern on the road. On the plus side, they roll easier. Basically, you prevent much of that sidewall-flexing that's so useful off-road (and can even preserve energy because the wheels don't bounce as much) but only seeps energy on-road.
But what about even higher pressures then? Well, you can go higher, and it may still improve rolling somewhat, but here we get into diminishing returns: even a hypothetical MTB tyre at 130 psi would obviously not roll as well as a road tyre, due to the knobs and aero. In fact it might well roll worse than at 60 psi, because it can't even take up the vibrations by the knobs and road surface anymore. Such pressures only really make sense for slick tyres on a velodrome's wooden planks.
So there's not really any reason why manufacturers of MTB tyres would rate them for more than 45 psi: all it would likely accomplish is that the people who misunderstand rated pressure as the pressure you're supposed to run would be even more dissatisfied with the off-road performance. And the occasional case where a tubeless pops off at already 50 psi (can happen due to tolerances) doesn't help either.
The one application where going significantly higher than 45 psi can make sense for MTB tyres are extremely hard jumps, freeriding. You know, the kind of thing like jumping down 5 metres and landing on a stair set. But that is a) dangerous at any pressure, and the people who do this sort of thing know that anything going wrong is their own responsibility and "rated" values are irrelevant b) even then tyre inserts are probably the better option for preventing pinch flats, instead of going to 80 psi or so.