The world is filled with situations where something with lesser economic value secures something with higher value. That's a sign of efficiency.
My house is worth $250K. But you couldn't capture that much value by breaking in, so I haven't spent $250K on a security system. If I did need another $250K to secure my first $250K investment, it wouldn't be a great investment.
Suppose it only takes $3.5B to attack a $7.3T network. So what? Unless your plan is to burn billions of dollars for funsies, you'll need to capture at least $4B worth of value for that attack to be worth it. You think you can find someone to anonymously make a $4B crypto sale to you, who won't know where to hunt you down when you try to 51% away the transaction after the next block in 10 minutes?