Because when buying alcohol, you can for the most part (outside of online purchases, but even then the courier can check an ID upon delivery hypothetially) verify it fairly easily. With online porn, the scale and complications doing it put a much bigger burden on the providers.
Additionally porn has a much bigger stigma than alcohol, so keeping record of what you watch, especially if you're into something more niche connected to your name can be used to discredit political opponents or even single out certain group due to abuse of power. A good example would be exposing a conservative politician that watches gay porn. Not that would necessarily be a bad thing, but there is no equivalent to this with alcohol - oh you drink tequilla, but you're whole ideology is based on drinking whiskey...
Additionally porn has a much bigger stigma than alcohol, so keeping record of what you watch, especially if you're into something more niche connected to your name can be used to discredit political opponents or even single out certain group due to abuse of power. A good example would be exposing a conservative politician that watches gay porn. Not that would necessarily be a bad thing, but there is no equivalent to this with alcohol - oh you drink tequilla, but you're whole ideology is based on drinking whiskey...