Because most users are on mobile devices and battery life is a limited resource. It isn't good for the user OR the business if their device dies mid-day and they stop consuming because you ran a miner in the background.
That can't be the only reason though. There are lots of things that are significantly worse/totally absent on mobile OSes already. Lots of software is made desktop-first, including apps like obsidian and (from the current HN frontpage) Arc browser. Mobile users have been 2nd class citizens in some ways for a while.
We were discussing Meta though. They absolutely depend on mobile, that is the majority of their user-base. In fact that is the majority on all ad-supported social media sites, so mining is a non-starter for them. Mobile traffic is >55% of all traffic and >80% for social media sites.
I'm not sure that the original comment was in context of meta or ad experiences in general online, but yeah I'd agree that meta needs mobile users a lot.