343 spent years developing a new engine, Slipspace. They even hyped it up with the release of Halo Infinite.

Then they told us they were moving to UE5, which pretty much cemented the fate of Halo. They could have spent time training people to use it, but considering they were doing that contractual work, then they would have to keep retraining them. That sort of development will never lead to a good game anyway, which is why we are getting remakes nobody asked for.