What it would take is a lot of work and time. And of course money to pay for all that work.

Something to keep in mind is that those people working on the hypothetical engine upgrade would then NOT be working on other things. And you can't just hire a bunch of new people and throw them at the task, unless you want the development to get very inefficient. What this means is that an engine upgrade would mean that regular content updates would get delayed quite a bit meanwhile.

As it is, I assume they actually do upgrade things here and there as needed and when they have time. Not a full rewrite, but many small improvements.