Earlier quoted context omitted.
You are still getting sued.
It's a theoretical benefit, not a real one, and the scenario is so rare that most people don't think of it critically. It's like buying insurance against "damage by asteroids". E.g.: "We can just sue Microsoft" is a common one. a) Their SLA literally states that they will compensate you only for the cost of the individual service they provided. The $1/month Azure DNS Zone outage caused your $100M/month business to va…
You have to decide, as a business, if trustworthy vendors are available and select the right one. You can also decide that it's important enough to your value proposition that you want to "own" it.
The real reason companies hire vendors is that you don't need some mythical "FTE" that can do all the things. For the "price of a single FTE", a vendor can provide you access to a range of FTEs that are knowledgeable and capable in different domains, and you can flex between them. Thinking about RedHat, as an example, if you pay RedHat the equivalent of a single FTE, let's say $300k/yr, is that just one FTE working on maintaining Apache? A kernel contributor? GCC?
This is about fractionalization and labor specialization, not some direct 1:1 tradeoff between hiring some mythical person with commit bits in hundreds or thousands of OSS trees.