That’s a bit of an exaggeration 
UE4 landscapes aren’t that low-res… there are plenty of good looking landscapes done in UE4… plus you can use tessellation (like most games nowadays probably do).
I would say performance is probably the problem and the editing tools.
Performance could definitely be better (use of better LODing system, quad-trees maybe?), the editing tools are either not good enough or completely useless in my case (if you want a proper terrain you absolutely have to use a heightmap generation tool like world machine), there is no voxel engine for caves etc. and no real-time editing or procedural generation.
I think the resolution of the landscape mesh is not really a problem… I suppose we could have better density control and height interpolation options but I would personally be far far more interested to see a voxel engine and real-time editing.