Because they would need to sell those shares for there to be a takeover.

But they already wanted to sell their shares to Nvidia before, what would have changed now? They want to reduce their ownership of ARM, hence them doing an IPO in the first place.

The regulators would stop it like they did last time.

Regulators stopped a coordinated purchase, which they are totally within their right to do. And if Softbank and NVIDIA try to coordinate a purchase another way, they'll stop that too.

It's not as cut-and-dry if Softbank just puts the shares on the open market, and NVIDIA pays market value for them. Are there reporting requirements around NVIDIA buying stock on the open market? Freeze-out periods? Regulatory delays? If there's collusion it might be a crime but even if you're cynical and refuse to believe that there might not be, you still have to prove it.

The antitrust laws and regulations don't change.