If I had to guess, its probably for two main reasons. First, I feel like the production companies want to wait as long as possible before renewing -- usually after they're sure the previous season is a success. Second, prestige TV in particular has increased production value past where movies were 10 years ago. More production value means more time.
It's more the first thing in many cases. Amazon, Netflix etc are green lighting new seasons and cancelling things based on spreadsheets at this point.
Look at two examples from my last work year. Bosch Legacy (essentially seasons 8-9-10 of Bosch, but pitched as a new show so they could cancel all the contracts of the writers and actors so seasons 1-3) and Ballard - a show based on another property from Michael Connelly, which Bosch Legacy season 3 episode 10 launched as a pilot.
They cancelled Bosch in like January or February or something when they were still editing the series because the spreadsheet said they would never get enough ratings to justify paying whatever bonus Titus Welliver was likely set to get in season 4. Ballard on the other hand hasn't aired yet and they will decide on based on the viewership.
Last year, I started filming Bosch in like April, then aftger a few weeks went straight into Ballard. now no Bosch over the summer and might not have a decision on Ballard to the fall. if they renew it, it will be breakneck speed to throw season 2 together because if they wait too long, no one will remember it had a season 1 to begin with.
The old way of having four or so networks with seasons that began and ended at the same time for everyone was much friendlier to the audience (and the crew.)
Now I can't claim I have ever worked on a prestige show like Game of Thrones, with a bajillion locations, but yeah they need a lot more time because the sheer reason that they have to get production from one place to another to film out all the season's worth of content for that location. I guess the closest thing to that which I have done was a limited series called "Murder at the Edge of the World." It was a bit of a clusterfuck in terms of the producing team, who weren't seasoned enough for what they were doing, which basically involved three big locations for filming: NYC, Iceland then Utah. they filmed the indoor Iceland stuff in NY so I had a lot of work for that bit, nothing when they were in iceland, then new crew members I had to deal with in Utah. It took so long to edit that show together I didn't think it was actually coming out and kind of thought it got shelved.
FWIW a typical 1 hour drama takes 8 days to shoot, but the recent trend is to do it in 7.5 days and have that half day line up with the first half day of the next episode. Will Trent works this way. it's a streaming show, but does 18 episodes and not all released at once, so it follows a more typical schedule. Bosch and Ballard still did the 8 days thing, but would sometimes jump around to film bits from other episodes. To be honest there was no reason that Bosch couldn't have been out in say January, but they were holding on to it to have it release closer to Ballard was ready. Every platform has its own reasons. I am sure Stranger Things is ready to go, but Netflix wants to ring all the life out of Squid Game Season II and Adolescence before they let it go free. The problem with binge culture is they have to keep feeding th ecouch potatoes to get them to subscribe for another month. They release too much at once and they are screwed.
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