Insomniac has been around earlier than Ratchet and Clank. They made the original Spyro trilogy, and if I remember correctly their office was literally in the same building as Naughty Dog's back in those days. In fact, there's a lot of easter eggs between the PS1 Spyro and Crash Bandicoot games that referred to each other specifically because of their close relationship (literally!). I think one of the Crash games actually had a playable demo for one of the Spyro games (or maybe vice-versa). You'll never see something like that anymore, so I agree with you that it is a surprise that Insomniac wasn't acquired by Sony sooner.
Sony's style of acquiring studios is, in my opinion, the right way to do it because let's face it: acquiring a whole studio is a BIG risk when you can have a rock solid developer-publisher relationship and reap most of the benefits instead. As soon as you acquire the studio, you have a lot more control over what they make but you also expose yourself to the risk of a flop since they are now on your payroll. Sony is super cautious and conservative with who they acquire, hence why their mantra is "Quality over Quantity". Because Sony does not have the financial power of Microsoft, they need to pick and choose studios that have proven track records and are a good cultural fit, not just with Sony but also with Sony's other first party studios. For Sony, once you're a Playstation Studio, you're family. Each studio helps out the others and there is a ton of collaboration. This approach helps the studios collaborate and not reinvent the wheel if another studio has already figured something out, which ultimately benefits Sony. For example,
Naughty Dog houses Sony's ICE team which was formed back in the PS1 days when they realized that a lot of what they were figuring out about the PS1 could be applied to help other first-party developers. ICE has been a crucial part of Sony's studios for every console generation ever since, and it's one aspect of the secret sauce that made Sony so successful. To use an analogy, Microsoft gobbling up IP as if it were at a buffet may work, but a good chunk of what they devour may be vomitted back out due to it not mixing well with whatever was already in its stomach or because there's simply not enough room in the stomach.