What is being bought here is the technology. Similar to any big VFX company, there is a lot of proprietary technology (renderers, modelers, engines, pipelines etc) that Unity is purchasing here. VFX tech is worth millions. Ask Autodesk or Adobe.
Some software like Nuke had good internals and had a lot of development put into a polished interface, which then payed off in a big way. I'm not skeptical that weta has some good internal technology, I'm skeptical that it is worth 1.6 billion dollars, especially to Unity.
I agree that the purchase price seems a little silly. They don't really have too much IP except for the in house tools, and 3d model assets.
I wonder how these assets will get licensed for use? Will it be a free for all (awesome!) locked under a pro/enterprise license (not bad) or under a license that will require a royalty-based payment to Unity?
If royalty based, the earnings from just the assets alone could be in the tens of millions a year.
Some of the Weta software is awesome technology. Weta has a good engineering philosophy, and the ease of use for much of thier portfolio means that you don't require much training to work with it.
When Epic spent 1.6 Billion they got FallGuys, a large studio, and a publisher.
Unity has taken on debt to buy the chance to payout yet more salaries. When Epic spends, they do so from profits, to buy more profits and IP.
The incredible irony being Wata's asset library will all be high poly, and thus the only video engine able to run them: UE5