I think it's easier for them to stay on the same engine. It doesn't matter how old some engines are, if they are updated and modified then there is no problem. Call of Duty is using the same engine since Modern Warfare (more than a decade), they are just adding what they need at each new game. Keeping the same engine allows companies to ba laz... I mean win some time by copy pasting the code from previous games (again like Call of Duty
). But as long as it works, I don't see any problems if they keep the same engine.
Sometimes keeping the same engine can fuck everything up. Take Bethesda and the Fallout series. The engine for F4 is outdated but its main strength is that it is very mod-friendly, allowing the modders to have access to good mod tools and a big sandbox to create a lot of stuff.
It's okay for Fallout 4, but they took the same engine for F76, added a multiplayer functionnality to their engine, and they copy pasted 90% of Fallout 4 and now their online Fallout have the same bugs as Fallout 4 on release. Too bad the community can't fix the game for them now
. This is a good exemple of why it wasn't a good idea to keep the same engine (even if the game itself wasn't a good idea).