The engine is THE major technical issue with their games. It isn't just about their games looking dated, but how they perform and what can be done with them.
Gamebyro reads every creature and NPC as a playable character. AI processing, rendering, tallying inventory, etc. That's why Bethesda cities are empty; the engine can't handle a 'real' population. Landscape editing is another big one. Gamebyro can't render terrain on the X or Y axis, only the Z axis (up or down). That means you can't sculpt rocks, mountains or caves, have to use actual 3D models and that taxes everyone's machine, no matter how powerful your rig is. Loading doors for cells, can't move from an interior or exterior without one, no ambient exclusions either without using image spaces, which is also a drain on your memory. No volumetric lighting so sun rays, mist, fog, dust, etc. are all done with meshes, so that's more geometry to render. No real water. Bethesda can't do any of this shit BECAUSE they're using the little engine that can't. And all of that is just how their games look and perform; it doesn't even cover script processes and animations.