And you get the IP as well, which is nice. In some cases you're consolidating power and shutting down competitors. It makes a lot of sense in some situations.
Unless you value the goodwill or IP of the target company, it makes no sense to buy out the original investors of the target company.
First: why would the investors of a company allow the team to be bought out from under them? A good investor will align the incentives so that the founders are incentivized to exit at the same time as the investor.
Second: why would a founder of a successful company accept a hiring deal? Most founders (heck, most developers) turn down several job offers before the second cup of coffee. And even if the $ paid to founders for acquisition vs pure hire is identical, there is an important psychological distinction between being bought and being hired that affects both the negotiations and the status and authority of the "employee" at the acquiring company and thus their wellbeing and autonomy. These factors matter.
My guess is that Google, etc, do this for anti-competitive reasons, as a way to keep engineer salaries from getting bid up. There seems to be a cultural stigma with paying much more to some engineers than others, which is why they don't just offer everyone at the target company a $250k signing bonus to come on board instead of cashing out the target company's investors.