It is a difficult question to answer. For example, why did Google acquire YouTube in the early 2010s? A platform technically and engineering wise similar to YouTube would have been very easy to replicate. IMO the best explanation goes back all the way to the days of Standard Oil/Carnegie Steel company - and quite possibly even the East India Company. There's an enormous benefit to consolidate various businesses under you and create a monopoly. Today in tech, monopolies are far from being as straightforward as being the dominant producer of a commodity like oil or steel. But there's undoubtedly some similar mechanisms involved. Synergy is one way to put it, but I think it's too restrictive.

I think the other part of the equation missing is if Google did create their own Wiz, Wiz would still be on the market, and it'd be a bitter fight which they could very well lose.