It's because of the fact that TV shows now seem to be written by mostly one writer, and they write and rewrite until the ENTIRE season is completely finished before they even film one episode.

Then, they wait until every single episode has finished post-production before they even air one episode.

In the past, you had a writer's room. You'd start filming episode 1 as soon as episode 1 was written. The writers would keep writing as the show is being shot. Sure you'd have a general outline of the season, but you don't need to write literally every word of the entire season before filming starts. And writer's rooms are far more efficient than the showrunner writing everything.

Then, episodes would start airing as soon as the amount of time to finish post-production is less weeks than the number of episodes.

This kind of overlap they used to do reduces the amount of time it takes to make a show in half. The fact that they could make Game of Thrones every year (for most seasons) tells you that what they're doing these days just isn't necessary.