Retros_x
Adventurer
The main reason is that most campaigns don't last long and most campaigns start at level 1 or 3. Because that is THE fantasy of D&D, from Zero to Hero. Seeing your character grow from a weak fighter to a mighty hero. Most people don't want to start at level 12. In theory almost everybody loves the idea to than grow the character from Tier 1 - Tier 4. But reality hits for most games that last that long.
I think its the other way around - because most games never reach that high level the content support is weaker for high level. Which makes sense from a business perspective. Why would you spent lots of money in designing content that most players will never see.
I don't think any DM wanted to run a high level campaign, started to prep, saw that the MM ist not appropiate to their needs and thus cancelled the high level game or stopped the campaign early for this reason.Want to run a campaign where your enemies are dominated by humanoids like I do and you have to constantly reimagine monsters or create your own which not everyone is good at. Add in lack of module support, which I can understand.
I think its the other way around - because most games never reach that high level the content support is weaker for high level. Which makes sense from a business perspective. Why would you spent lots of money in designing content that most players will never see.