Let me add to this: Earnings can only be moved to credits when making a purchase. It is not possible to move all your earnings to credits and then purchase something from that at a later time.
If you do not want to get a payout you will have to spend enough of your earnings so that you are under the payout threshold at the end of the month.
Not setting a payout destination does work as well - however, Envato may reach out to you and ask you to set one, maybe close your account if you don’t do so.
This is guessing:
Envato does not want to hold a lot of money in their accounts because that can have legal implications of them being treated as a bank and not a marketplace. So they rather pay you out as often as possible than keeping the money.