Because computers have come even close to needing more than 16 exabytes of memory for anything. And how many applications need to do basic mathematical operations on numbers greater than 2^64. Most applications haven’t even exceeded the need for 32 bit operations, so really the push to 64bit was primarily to appease more than 4GB of memory without slow workarounds.
I’ve never seen any math that disproved that. Now if you’re looking at actual federal income tax paid, maybe, but social security tax is based on income, it doesn’t get reduced by often fraudulent deductions and credits. So a lot more income is taxable under social security tax than federal income tax which is laden with loopholes and complexities. So when Jeff Bezos in 2007 paid $0 in income tax, he likely would have paid tens or hundreds of millions in Social Security tax if there wasn’t a cap, because he did have income. Of course, we aren’t legally able to get the actual numbers because of privacy laws, but there have been some leaks recently if you want to see some of the numbers.