We pay money online mostly through credit cards. Credit card transactions can be reversed. If children spend money on porn, those payments are likely to be reversed. This is really bad for the ability of the porn sites to continue receiving credit card payments, and continue making money.
An age header is a trivial step that can reduce the odds of the adult site receiving payments that later get reversed. Win, win.
But if someone is willing and able to pay, then the adult industry wants the choice of whether to access content to be up to them. If government tries to regulate them, they'll engage in malicious compliance - do the minimum to not be sued, in a way that they can still reach customers.
For example Utah tried to institute age verification. The porn industry blocked all IP addresses from Utah. Business boomed for VPN companies in Utah. Everyone, including porn companies, knows that a lot of that is for porn. But if you show up with a Nevada IP address, the porn's position is, "You're in Nevada. Utah law doesn't apply." Even if the credit card has a Utah zip code.
If you live in Utah, and you're able to purchase a VPN, the porn companies want your money.