But everyone still has to go through the steps of getting their money on to those alternate chains, which isn’t free. All of the transaction costs and exchange fees leading up to the checkout transaction and when the vendor later gets their money back out of the cryptocurrency are still expensive.
The only way vendors come out net ahead by using cryptocurrency is if it affords them some additional protections at the expense of the customer. For example, forcing customers to eat the transaction costs and exchange fees, eliminating the possibility of chargebacks if the product or service is bad, and so on.
Or more simply, they use cryptocurrency to dodge taxes. Or try to, anyway.
I'm only aware of Dharma wallet, but I'm sure there are others - allows direct & free(well, except MATIC for gas I suppose) deposits from your fiat accounts onto Polygon, an Ethereum L2.
Your exchange should support the alternate chains. They get economies of scale with the gas fees when bridging between chains. Of course, relying on an exchange for this contradicts the decentralization ideal.
The only way vendors come out net ahead by using cryptocurrency is if it affords them some additional protections at the expense of the customer. For example, forcing customers to eat the transaction costs and exchange fees, eliminating the possibility of chargebacks if the product or service is bad, and so on.
Or more simply, they use cryptocurrency to dodge taxes. Or try to, anyway.