Why can't they just land on their floor? If it's for stability, they could still use skids which are less taller than what is used.
Larger helicopters have landing wheels, and as we can see on the picture above, they are placed as far outward as practically possible in order to provide stability in the landing. Touch down one wheel on an sideways inclined slope, lower the collective, and the helicopter will align itself onto the slope with:
- The location of the Centre of Gravity (CoG) always in between the wheels. The higher the CoG, the wider the landing gear impact points must be.
- The weight being supported on clearly defined and structurally reinforced points. Even if the bottom of the helicopter is perfectly flat, the landing ground may not be, and to support all of the weight at any random point at the bottom requires a lot of relatively useless weight for the re-inforcement.
- The behaviour after first wheel touchdown being clearly defined. The wheel is a point of impact upon the landing plane, mathematically much more defined than a plane of impact upon a landing plane if the latter is curved.
All the above is for wheels, which allow the helicopter to taxi on-ground. Skids perform all of the above functions apart from the on-ground taxiing, and are cheaper and lighter than retractable landing wheels. Of course, the other downside is that they have more drag.
One would not want to land this helicopter without the skids!
Physically they likely could, but helicopters (and all aircraft for that matter) tend to have stuff mounted to the bottom like antennas, probes, cameras etc. so adding some kind of landing gear or skid helps to provide clearance for all that. This question covers a bit about whats mounted where.
The belly of a helicopter also tends to not be perfectly flat (depending on the model) so belly landing can lead to some roll which may allow the rotors to scrape the ground.
Skids are especially beneficial when the helicopter is supposed to land on rough ground, e.g. grass.
They move the body and the rotors farther away from the ground, thus giving both more clearance.
Compared to extending the body to the ground to achive the same effect, skids add less weight and significantly less air resistance.
Compared to wheels they are better suited for rough ground, as they have a larger contact area and thus tend to sink less into the ground.
Wheels on the other hand have the advantage when the helicopter usually lands on concrete or other solid ground. Here they allow taxiing.
That's why you tend to see skids on smaller helicopters which tend to land on grass or other uneven terrain, while larger helicopters, which tend to land on more solid ground, tend to have wheels.
Also, skids are simpler than wheels, which also suits smaller, cheaper helicopters better.
Skids, as opposed to wheels, allows the pilot to get some sense of balancing tail rotor counter torque before lift off. Skids would provide more drag until the pilot "got it". Same thing moving forward, backwards, or side to side. The skid friction limits movement (or damps it if you will), rather than creating a nightmare of correct/overcorrect in 2 dimensions.
Skids also add an important amount of rotor clearance, and offer simplicity, reliability, and light weight.
An additional benefit of longer skids is some energy absorption upon a vertical crash. The skids are generally designed to bend outwards and up during impact. This can be seen occurring on many helicopter crash videos. The design of impact absorbing skids is covered extensively here.