They don't. It can feel like they do because the same libraries, models and shaders are reused a lot between games, but there's a lot of variety if you look hard enough. Comparing the look of RiME to Crackdown 3 to EVE Valkyrie shows there can be a lot of difference.
Have a browser through Unreal Engine's trailer showcase - https://www.youtube.com/playlist?list=PLZlv_N0_O1gYaMTM__mGE...
But then there's the Borderlands games. Those are based on Unreal, but they don't look like any other game out there.
A newer game that has pretty decent lighting is PlayerUnknown Battlegrounds. Check it out, you might see less of those aggressive reflections.
There will be moments where the landscapes and vistas are almost as impressive as Witcher 3 and then I get shot.
I think the possible explanation on why they may look different to you than other PBR engines (say renderman or unity) is that IIRC UE4 has slighly different curves for roughness and other properties. but still I would credit it to what the other comment said about shared assets and people using defaults.
you can see information from disney here: https://disney-animation.s3.amazonaws.com/library/s2012_pbs_...
and here are the ue4 docs, with some other references at the bottom: https://docs.unrealengine.com/latest/INT/Engine/Rendering/Ma...
If they aren't straying far from the defaults then it should be obvious why they would look the same.
While an experienced Unreal developer knows what to look for, the average gamer most likely couldn't tell if a AAA game was using Unreal Engine 4.
Still amazing tech, just something I've noticed.
My thought it is because they use the default shaders (and/or the artists do glossy too much). That said..
Most, if not all, modern big engines use "deferred shading" and support normal/bump maps, gloss/reflection maps in addition to "normal" textures (lately every rendering engine that focuses on being realistic is switching to "Physically based rendering", that is very similar to "normal" rendering except that it obeys the "amount of (light) energy out can not be greater then the amount of energy in"(this also means linear light space, instead of logarithmic like we humans perceive)).
All the big rendering engines can, with the same equations in shaders, produce exactly the same result. The same results in regards to the direct "shading".
The other part of how a thing looks is lighting (aka "global illumination"), where the actual engine code is a lot more involved. Since you can't make, like raytracing does, light bounce off of things, you have to cheat (it uses too much processing power). Rendering engines cheat by, for example, plunking "light probes" around (unity had a good post about them, i can't find it now). The slight differences in lighting may not seem like much in a side by side comparison, but our brains use those little lighting clues meaning they(we) are sensitive to them.
There is also how many lights a rendering engine supports (and the distance they can cast light), how it does particles and can they emit light, how it does volumetric.. things (like smoke, maybe water and such), how good it is at reflections (are they precise, do they include dynamic things; note that they probably don't bounce twice), subsurface scattering (notable on skin), and probably more that i forgot.
Note that i simplified plenty. For example lighting and shading are closely related.
PS Reality is not like basic theory. Physically based rendering does give real world results but in reality everything has dust on it, or an oxide layer (aluminium especially), or a sulfide layer (copper likes sulphor). The other imperfection is the minor imperfections in smoothness, what the collective "they" do take into equation (see https://www.cs.cornell.edu/~srm/publications/EGSR07-btdf.pdf).
PPS "Physically based rendering".. as if Phong shading is based on something else. (PBR is still no where near reality, for reasons above)
The same with ID Tech, Unity, RAGE and CryTech/Dunia games. One can spot them "from 50 feets away", or at least when trying out the controls, the character physic gets rarely changed at all. (I cannot say the same thing for Frostbite 3 and AnvilNext, the are more modern.) And many Unreal and Unity games use the standard shaders included with the dev kit.
(e.g. Titanfall 2 based on Source 2 still feels very like the controls from Quake 1, well it's based on the same engine after all).