Exactly!Maybe not, but they're designed to be. They always have been, too.
Old D&D capped out at 6th level spells. 6th level spells are the first spells you get beyond name level. They used to be so special that you got 5th level spells at level 9, and then you got 6th level spells at level 12. B/X also capped at 6th level spells, and that went to 14. It was in many ways supposed to be the endgame spell level. Spells for levels 7-9 are stuff that happened much later. In nearly every AD&D campaign I played in, 7th level spells and higher were the exclusive domain of NPCs. And, realistically, that's how the game was designed. The XP tables got too fat at high level to realistically keep adventuring.
Even in 5e, 6th level spells are designed to be endgame stuff. It's the first spell level Warlocks get that isn't short rest recovery. It's the first level of It's the first spell level that doesn't really increase in daily use. Spell levels through 4 and 5 quickly ramp up to 2 or 3 per day. You get your first 6th level slot at 11, and your second comes at level 19. 6th level is also where nearly all spell level recovery and spell storage items end. Wizard's Arcane Recovery and Circle of the Land Druid's Natural Recovery cap at 5th level. It's the first spell level that Sorcerers can't create with spell points. Ring of Spell Storing is up to 5 levels. The game goes out of its way to prevent any one character from getting a second 6th level spell for a very long time.
The fact that the spells themselves at 6th level are almost universally lackluster, especially compared to how bonkers insane so many of the 7th level spells are, is kind of irrelevant to the design of the rest of the game. Everything in the game except the actual spell descriptions themselves treats 6th level as the start of something different.
The truth is just that spell descriptions are undercooked in 5e. It's pretty clear they didn't spend very much time on much of anything after level 11, and essentially nothing after level 13. The design of the game is set up for 6th level spells to be singularly amazing. They just didn't actually do that. They moved all those singular effects to 7th level, which is why level 13 is when the game stops being much fun.
It all seems like such a clever system, where the designers recognized the pattern of power spikes in D&D's inherited spell list, and then benchmarked the power of everything else to those levels--with attack and cantrip progression, pact magic, font of magic, and arcane/natural recovery all built, in part, around that 6th spell level power spike.
But then they nerfed a bunch of the spells, like disintegrate and flesh to stone, that made the 6th spell level powerful.
Feels like the remedy would be to just make some lackluster 6th level spells stronger.