Because fabs are about the most complex cutting edge technology out there: the "rocket science" of our day (or one of them). And merely having the money is not sufficient. It would be very easy to blow several billion dollars and end up with nothing to show for it.

Just look at how Intel has struggled to compete in recent years, and they have been in the business for decades.

Intel struggled because they bet the company that Moore's law was over back in ~2014, and instead of upgrading their fabs to EUV they sent the money back to shareholders.

They forgot Moore's main lesson: only the paranoid survive. They thought they could coast, and it nearly killed them.