it's a bigger investment, but it's an investment which will pay dividends for decades. with a compute cluster, the government is taking on an asset in the form of the cluster but also liabilities in the form of operations and administration.

with a fab, the government takes on either a promise of lower taxes for N years or hands over a bag of cash. after that they're clear of it. the company operating the fab will be responsible for the risks and on-going expenses.

on top of that...

> thousand of staff

the company will employ/attract even more top talent, each of whom will pay taxes and eventually go on to found related companies or teach the next generation or what have you. not to mention the risk reduction that comes with on-shoring something as critical to national security and the economy as a fab.

a public-access compute cluster isn't a bad idea, but it probably makes more sense to fund/operate it in similar PPP model. non-profit consortium of universities and business pool resources to plan, build, and operate it, government recognizes it as a public good and chips in a significant amount of money to help.