Unreal | Engine 4.6.1 Compiled Installer

The most common reason is maintenance. If a studio shipped a game in 2015 or 2016 built on version 4.6, upgrading the project to 4.27 or UE5 is not a trivial task. The "Forward Rendering" changes, physics engine updates (PhysX to Chaos), and UI system shifts (UMG changes) would break the project extensively. To fix bugs in a shipped game, developers must open the project in the exact engine version it was created in.