A studio's core strength is usually creative: game design, art direction, narrative. Engine-level engineering, multiplayer infrastructure, and cross-platform build pipelines are a genuinely different discipline, and building that expertise in-house from scratch is slow and expensive for a team whose real value is on the creative side.
Where outside engineering partners add the most value
Multiplayer networking, performance optimization across device tiers, and AR/VR integration are all areas where deep, specific engineering experience shortens timelines meaningfully, letting a studio's core team stay focused on what makes their game distinctive, rather than reinventing infrastructure that's been solved many times before elsewhere.