Because to me it just feels like it might be legally separated, but still owned and directed by the same handful of people. And it being separated makes it safer, in that they can't forward e.g. large fines to the parent company.
Disclaimer: I don't know anything about large corporations. or economy. or governments.
The US did this with Standard Oil in 1911, Bell/AT&T in 1983. And the same laws were used against Microsoft in 2001, though the company was able to avoid a break-up.
Breaking up Google might not be the best option. Perhaps more rigorous regulation by the government would be better, similar to Microsoft. But a break up should be an option.