I personally believe there are two main reasons. One is GM burn out especially for games with complex rules like DnD5e the second is that long campaigns are inherently unnatural as most stories are relatively short and have well defined beginning middle end structure. If you look at the vast majority of media (movies, TV, novels, etc) “campaigns” are generally pretty short and the long running multi season/novel/-verses are far less common. I believe natural stories tend to be shorter and move faster through the plot than people realized. I like to look at media and break down how many scenes they would be at the table and most of them are around 4-6 per modern short (10 episode) tv season, novel or movie, that works out to about 3-4 sessions (a couple hours each YMMV). Building a long campaign requires 100 maybe 200 scenes and 10 or 15 novels worth of material, thats a lot more than most people realize going in.