[I] the DOF is shallower at longer focal lengths (if it really is)
and not just blindly accept it.
a) A lens is a combination of various glass elements arranged in groups to achieve a certain angle of view.
b) Prime (fixed) lens have one specific focal length, e.g. 24mm, 35mm or 50mm, or 100mm, each offering a specific angle of view.
c) The longer the focal length, the narrower the angle of view, e.g. 24mm offers a 'wide angel view', a 100mm offers a 'narrower telephoto view'.
d) Now, one more factor, is the aperture; the bigger the aperture opening, e.g. f1.8 the brighter is the image and hence allows faster shutter speeds. If the lens is e.g. f4.0, the image will be dimmer, so a longer shutter time will be needed for correct exposure assuming same ISO setting. The f-number is a mathematical relation between diameter of aperture opening to the focal length of the len, e.g a 50mm/f2.0 lens means the diameter of the aperture blades is 25mm while the lens focal length is 50mm.
e) Finally, the DOF is also a mathematical relation (law of optics) and are influenced by 1) aperture size 2) focal length 3) subject-to-lens distance:-
f) f1.8 will produce shallower DOF than f5.6. f5.6 will be shallower than f11.
g) a zoom lens shot at 200mm will have shallower DOF than at 70mm.
h) DOF would be shallower if the subject is shot very near the lens as compared with greater subject-to-lens distance.
IT'S JUST LAW OF OPTICS. Lens designers use various knowhow to try and make the lens easier to use, but they all have to work within the physical laws of optics.
I try to be as 'untechnical' as possible but this is the best I can do to help understand DOF.