Because breaking companies up seems to be more theatre and less useful outcomes. People call for it freely and usually don't have a theory of how it will actually help. And nobody seems to bring up evidence from past corporate breakups beyond "we did it ergo it was good" (which is suspicious logic).

On paper it looks something like a choice between one company with one shareholder or two companies with ... the same shareholder. It isn't clear what has really changed; the company was only ever a legal fiction anyway. If there were incentives to collude then they haven't gone away.