<p>In every 3D print that I have seen, the bed should be leveled (manually or with some sensor-based system) on multiple points, pretty annoying because often when you reach the perfect distance on a point another point should be adjusted again repeating the procedure multiple time to have perfect leveling on all points.</p>
<p>I'm wondering why height isn't just fixed with the optimal leveling.</p>

The point of "leveling" (tramming) the bed is to make it:
square with the coordinate system of the printer so that it lies in a plane perpendicular to the direction of Z travel, and
a known distance from the nozzle tip at one point (and thus, due to (1), all points) in the plane at any given Z value.
In theory, if the machine were all preassembled and sufficiently/perfectly rigid, this could be done once at the factory and never need leveling again. However, even if that were possible, there's at least one factor that will throw you off: you're going to want to swap/replace nozzles, and they will not all be exactly the same length - especially if you use different nozzle diameters, styles, materials, from different manufacturers, etc. This is why, even with an ABL system, unless it performs probing using the nozzle tip itself, you always have to calibrate out the offset between the probe and the nozzle tip.
In practice, many machines (especially affordable ones) require final assembly by the consumer, which necessitates calibration. They also have parts which can warp or wear and require adjustment - especially POM V-roller wheels - which will then throw off the squareness of the bed with the rest of the frame.
If you find you're having to re-level often, something is wrong with your printer. You may have play in the Z-axis/gantry - on Ender 3 style printers with only one side driven, the undriven side tends to wander up/down unless you get everything tensioned perfectly, and this manifests as apparent loss of leveling. You may have
adjustment wheels working themselves loose
(watch the end to see final findings) that mess up your leveling (I've had good luck mitigating that with
this add-on
). You may have an unreliable endstop switch (mine was acting up recently after years of use, and replacing it fixed the problem entirely).