TL;DR Microsoft has a strict policy of only hiring contractors to work on Halo, and only keeping those contractors for 18 months before they’re forced to go. Something something Microsoft execs want to be able to fire people.
One thing to note that seems to be missing from the article: Halo Infinite uses a custom game engine (Slipspace). So when these contractors are hired, they spend so much time training that they don’t end up working as a skilled employee for very long before they leave, then new contractors are hired.
It’s so stupid since this is so obviously a lose-lose situation: the contractors gain a skill that is useful literally nowhere else, and Microsoft’s games just keep getting shittier.