Suppose you're Coinbase. The way you protect yourself is on large transactions, waiting for more confirmations. This increases the amount an attacker must spend to do a 51% attack. If you wait long enough, it's not worth it. But wait! The attacker can use multiple accounts and multiple exchanges, so now Coinbase has to look at the total volume of newish transactions on the blockchain in order to know when it is guaranteed to be safe.
The upshot of this is that a POW blockchain cannot securely transact more money per hour than the 51% attack number. That's also approximately the amount of money miners spend per hour.
Total world payment volumes are on the order of a quadrillion dollars per year. Unfortunately world GDP is only $80 trillion. Even if we reoriented our entire economy to do nothing but mining, we could only protect about 10% of all of the transactions.