Why do distant objects pop in?
Because the game is deliberately not rendering things at full detail until they matter — and the pop is the moment a substitution becomes visible, which means it is a performance technique showing its seams rather than a bug.
The techniques producing it:
Level of detail. Each model exists at several complexities — a full-detail version, then progressively simpler ones with fewer polygons and smaller textures. The game swaps between them based on distance and screen size. A visible jump is an LOD transition that was not blended smoothly enough.
Culling. Objects outside the camera's view, or hidden behind other geometry, are not drawn at all. Occlusion culling is conservative and imperfect, so something can appear as it becomes visible.
Streaming. In an open world, assets are loaded from storage as you approach. If you move faster than the streaming system expects — a fast vehicle, a teleport, a fast-travel arrival — you outrun the loader, which is why pop-in is worst in fast travel and flight.
Texture streaming is the specific case of surfaces appearing blurry then sharpening, as a higher-resolution version of the same texture arrives.
Distance fields, imposters and billboards, where far-off trees and buildings are flat images or simplified stand-ins that switch to real geometry up close.
Shadow and lighting cascades, where shadow quality is banded by distance and the band boundary is sometimes visible.
Why it is necessary. Rendering everything at full detail would require orders of magnitude more computation, memory and bandwidth than any hardware provides. The question is never whether to substitute, only how well the substitution is hidden.
How it is hidden well: cross-fading between detail levels; geometry that morphs continuously rather than swapping; fog and atmospheric haze, which are as much a technical tool as an artistic one; and generous loading margins.
What you can do: raise view distance and texture streaming settings if you have the memory and bandwidth; install to faster storage; and note that higher frame rates can make pop-in more noticeable by revealing transitions that motion blur once concealed.