2D or "flat landers" only exists as a concept to try and explain higher spacial dimensions, the smallest distance we have is the planck length, so the most perfect square you can ever hope to make will still be at least that high. meaning its a cubic shape

4D, or more the manipulation and translation of 4 or more spacial dimensions is where we come unstuck, as we lack any good reference point for it, so trying to approach it from a lower dimension, where the thought of being able to see all the 2D slices of an object as a 3D shape is easier to grasp than thinking that you represent a 4D shape with 3D slices,

Its not impossible to learn, but its very hard to visualize, kind of like how black holes are technically points with no dimensions, the have no width, height or length, only a location in space, its not easy to wrap our heads around, e.g. you have 100 millions suns mass stored in a point with 0 volume,

Then higher and higher counts of spacial dimensions, and even the concept of a second independent time dimension are thrown in the mix to try and explain things that may be results of us struggling to visualize it, (String theory and its friends)

There are even fun things like the weak force, where how it reacts does depend on its "spin" or orientation, and even which way time is flowing, so you even have physics that depends on how many dimensions there are.