Because in the short term it would disrupt a major company (ala Standard Oil), but in the long term it would allow the US to remain competitive in the global market.
If we allow Google to continue abusing its monopoly power in the US, that guarantees that the US will not be the home of the future technologies of the world. Innovations will be sucked up and killed as acquisitions. Enormous energy will be focused on blatant moat-building like WEI instead of developments that benefit the world. etc.
Any successful US-based tech post-breakup would be acquired by larger international players, like Tiktok was.