The problem is the government has to acquire the hardware. There isn’t 51% of the hash power in spare mining equipment lying around. It’s all decentralized.
You're assuming you dont have state sponsors heavily invested in it already - and have been for years. Which is actually kind of hilarious, as it also assumes the blockchain is safe from other threat models. I wouldn't be surprised if a significant chunk of bitcoin movements were state sponsored; its the next logical move - first you build a brivate internet, then you build a private currency.
The US government could spend trillions to surreptitiously manufacture extremely specialized equipment without anyone knowing, turn it on to do a 51% attack, and when it’s over the network will just fork and reverse the transactions. It’s illogical
There's also a legal risk for private entities, as the conviction of Avraham Eisenberg showed recently, and I think a strong possibility that a successful attack would crash the valuation of BTC and make it impossible to recoup costs.
If BTC's marketcap is $1T and it'll only take $5B for a 51% attack, there can be some arbitrage opportunity for a large hedge fund determined enough.
The only thing preventing this is the attacker must execute this flawlessly including attacking all the forks that may happen.