Authors are human beings, and human beings have all kinds of reasons for failing to meet deadlines from writer's block to catastrophic illness. As others have said, they aren't machines. And gods help anyone who hopes to make a living off their own creativity when and if we do get to where a publishing company hires programmers who create algorithms that generate mechanically plotted series written in a smooth, agreeable style on a predictable schedule.
GRRM is an extreme case, and I have pretty much assumed the end of ASoIaF is not ever going to come, aside from what was written for the TV show. It's easy for me to say this, perhaps, because I lost interest in the series after book 3 or so anyway, and I only watched part of the TV series (not having HBO, I had to catch episodes when they had "free" weekends). It's a bummer for those who are more avid fans, but I can see why it would be almost impossible for him at this point. Maybe he'd have finished it by now if he hadn't signed a contract for a TV show before the series was complete, but I don't blame him for that choice either. I doubt I'd be able to resist if someone waved that kind of money in my face for an unfinished series that was popular before but hardly a household word for those who don't generally read fantasy.
I certainly know what it's like to be soured on something I once was enthusiastic about to the point that trying to continue with it almost makes me physically ill. I don't know GRRM, so there's no way I can speak for him, but I imagine if I were in his shoes I wouldn't really feel like ASoIaF was even mine anymore.
Having said this, I get why fans feel frustrated with waiting for something they felt was promised and wasn't delivered. We live in a world where we've gotten used to instant gratification. We get pissed off if we go to the grocery store and they are out of our favorite cookies, and hell hath no fury like mine if something takes longer than two days to arrive from Amazon. Writing takes time (I can think of a few authors who say they can bang out a new novel in a weekend, but I think they are unusual, especially in a genre that now expects doorstop-sized mega epics with involved world building). Still, some authors (and their publishers) have somewhat adapted. So a sequel often comes out a few months (sometimes sooner) after the previous book in a series, especially with newer or less famous authors. Some now publish shorter works at more frequent intervals. There's a marketing model at work here, because if a reader is waiting for a new installment that doesn't come soon, they get bored and distracted and "forget" what they were so impatient for. They may not remember how eager they once were for the sequel by the time it comes out because they've moved on to other things.
But this approach doesn't work for all stories and authors. Real life is messy and unpredictable. We can't really count on everything happening according to plan, and people needed to remember that even before we had a global pandemic.