It's not instant. According to https://www.crypto51.app/ the cost to 51% attack ethereum for 1 hour is $92k. This means it's $2.2 million for one day or 24 hours. Let's say you want to transfer $10 million via ethereum. Then the party who gives you the eth can perform a double spending attack, only pretending to give you the $10 million while getting something else from you in exchange as part of the original deal that takes one day to be transfferred to them. As long as that something else is worth more than $2.2 million, that other party has made a return. The only good remedy is waiting for the double spending cost to exceed the contract volume, but it means that the currency isn't instant any more, or at least is instant for small sums of money only.

Bitcoin requires a validation cycle or two in order to be certain that your transaction is going to be on the main chain, so it's 10-20 minutes.

For traditional infrastructure, the RTGS (real time gross settlement systems) accessible to banks and large customers handle transactions in seconds. If you're in a treasury department of a company who routinely needs to move large amounts around, doing so nearly instantaneously is absolutely a solved problem.

Also crypto isn't instant. Bitcoin takes 10 minutes if you're lucky, even longer if the network is maxed at 7 transactions a second (comparatively, Visa handles 42,000 transactions a second).