The vast majority of Dungeons and Dragons 5e games end around level 10, so this is actually the norm for tabletop. It can take months and months, even years of real-world time to reach that level bracket.
It's also when spells start becoming outrageously powerful. It's difficult to add a spell-like Shapechange to a virtual game, as it is a spell that lets you turn yourself into almost anything, including high-level monsters like dragons. How would you balance a wizard being able to turn the entire party into elephants and then stampeding over their foes? Or create their own pocket dimension? Or Heck, being able to summon a literal mansion wherever they want?
Adding Seventh, Eighth, and Ninth level spells would almost definitely be more complicated and expensive than adding in the branching plots you think are useless.
