Yeah, I've seen a fair few people complain about this, and they do bring up a fair few good reasons:
01. Loss of Momentum/Fan Enthusiasm
In the past, you'd usually get shows - or at least US shows - putting out around between 20 to 24 episodes a season, usually released weekly so it would last around 6 months, and you'd get a new season of it every year if it was successful, meaning the next season could start just 6 months after the previous one concluded.
As such, by the time the hype for the previous season was dying down, it'd kick back up again with the news of the next season and people excitedly discussing what they thought/hoped would happen in the show and what would be next for their favourite characters.
It kept everyone invested, helped fans to keep clear on what had happened previously and ensured good (or at least decent) viewing figures when the show returned.
Nowadays, a fair amount of TV series seem to getting chopped down to a meager 8 episodes or less - so they feel more like a miniseries - and yet somehow it's now taking those in charge years to make new series despite having far less material to film.
As such, the show's momentum ends up falling off a cliff and plenty of fans get bored and move onto other things; especially with people having much shorter attention spans, YouTube and numerous streaming services all trying to get their attention with new movies and shows (combined with the releases of new books, games, etc.), along with there been plenty of older books, shows, films and games that audiences can catch up on.
We all have a finite amount of time and attention, so the long waits between seasons don't help in keeping us invested before the next one comes along. For the fans who truly remain dedicated in their wait for the next series, said series still has to spend time reminding other viewers who watched the show in the past and decide to check it out again after seeing it advertised - because "Oh yeah, I vaguely remember that show; I wonder if it's any good now?" - of what happened previously just so they can be caught up.
It's for this reason that shows like House of the Dragon lost so many of their viewers between seasons; people get distracted and move on because it just takes too long for them to stay interested in what comes next.
02. Aging of Actors
The problem with the longer time it takes to make shows nowadays means that a lot of actors are clearly getting older, and it's especially noticeable with a lot of younger actors.
Bringing up Stranger Things since Princess Viola brought it up, that show first aired back in 2016 yet it's taken nearly a decade just to get to Season 5. As such, that "plucky band of kids/teens" are quite clearly a lot older now and it breaks the immersion badly when its still supposed to be a group of teenagers yet they're starting to look like they're in their 30s.
Granted, there's been plenty of movies that have used older actors in the role of school/college kids (a good example been the 1976 film Carrie), but at least most of them looked the part and were just there for the one movie; a TV show is a different kettle of fish and can be very off-putting when what you see and what the show is trying to tell you don't match.
If this show had been made using the original model of making shows, it probably could have been finished by 2021 or 2022 at the latest. As is, it isn't, and it's only causing more issues now.
03. Lack of Episodic Storytelling
While this obviously isn't something that can be done by all shows, a lot of shows in the past had episodic storytelling; no singular path to a grand finale but rather just individual adventures and self-contained stories that you could put on and watch anytime. Some were weird and wacky, some creative and thought-provoking and some just simply enjoyable and entertaining without trying to be anything grandiose.
It meant the show's writers could have fun with different ideas and themes with less restriction, while giving the audience a variety of episodes to choose from without the show becoming stale. Not every episode was good in that regard (especially those whose plots basically resulted in the episode been a clipshow, or sometimes when one of the actors wanted to write/direct an episode and it didn't work out), but for every "clanger" you at least had plenty of other episodes to choose from that you did enjoy.
On top of that, since there was no over-arching storyline, there was less worry about missing out and kept the show interesting.
Now, every TV show feels like it has to serialized and connect to a single over-arching storyline; meaning you can't afford to miss an episode because you might miss something important.
It stops being entertaining and instead becomes boring at best and exhausting at worst to the point of feeling like the TV equivalent of 'homework' - especially with the shorter seasons and the length of time taken to make new ones these days; it falling back to the earlier point of the next season then needing to remind everyone who decided to come back of what actually happened previously.
There's obviously others, but the three reasons I've talked about above are the main issues.
