Two old red dragons engaging a party of 7? I would say they were pretty confident. It's not easy to spot that somebody are level 16 or whatever, especially when they start of with level 1-4 spells.
Why are they bothering to engage a party of 7 tiny little humans? Presumably, they have some horses, so maybe there's a nice, light snack in there. And, being Evil, terrorizing the little guys may be fun. They're overconfident by nature, so swooping down to melee makes sense. Let's go with a nice, light snack combined with possible treasure and just being cruel by nature. However, I add the caveat that this makes them a random encounter, not an important aspect to the story, and those should be more speed bumps than dangerous challenges.
Now, there were two dragons coming in on us, and we spotted them about 3 dragon flight moves away, or 12 seconds of flying. They double moved the first round and used a move action the second round and were on us. We had a bit of an oversized party, so you had one wizard blasting and two clerics buffing in addition to the archer pelting them with arrows. Dragons are ok at strafing, but not great, so to get off a full attack, they have to get into melee range or just fly away.
So, you spot them 1,200 feet away. Not bad. Were you actively looking for things in the sky?
The fight, in typical DnD 3.x fashion lasted something like 4 rounds, including the two buffing rounds. One of the dragons engaged on round 2 (hovering to get of a full attack the next round close to the ground) and the other was a bit more cautious and engaged on round 3 (hovering in the air).
So you see them what, 1/3 of a mile away, and they can't see you casting spells (whoa - magic - reassess tactics!). And you are using your actions to buff. Did you not dismount? Were the horses not affected by the Frightful Presence of the Dragons? If there are no horses, they also spotted these little tiny bipeds from 1/3 mile away and chose to attack?
So they both take the Hover feat, not a combat feat? No spells cast at them in those first two rounds? OK, Round Three the bipeds attack and, surprise, are highly effective. So the first one stays in melee and the second drops down to engage? Not the tactics suggested by a 20 INT, 21 WIS, 20 CHA to me, at least. They use strategies worked out ahead of time, and that's the best they can come up with? Sure a full attack is better that a flyby - but it doesn't expose them to counterattack.
Neither thinks to Crush the little creatures so they can't run away? Its 11th level sorcerer spells are useless? It's too stupid to fly away and strafe after taking 3 digits of damage in six seconds? Seems like the PCs get all the benefits of tactics and planning, but the 20 INT, 21 WIS dragons just fly into the grinder.
Why can't the dragons also have Haste? After all, a 20 CHA Sorcerer has over 30 spell slots - he can never use them all. Slows and Holds to keep those soon to be panicked hors d'ouvres still seems like a reasonable tactic. The latter can be cast from 200' away, so the Hovering second dragon has something to do. They're still moving? A hint they are more powerful than first believed, perhaps? Revise tactics? Dispel Magic (also from 200' away) a few times? Use it to Counterspell (to cruelly frustrate and terrorize the little mammals)? What - they still cast the spell? Another clue they are a powerful force, and we should revise our tactics.
If the monsters used tactics half as effective as PC's they'd become more a threat worthy of two CR 20 opponents (which should be expected to use 20% of the resources of a party of 7 L20 characters -not tear through them like a hot knife through butter) and less a speed bump generously provided to a group of buffers.
They're malicious, cruel and overconfident. Why close right away? Why not let these mammals stew for a bit - fly away behind the mountains and come back a bit later - do you let those buffs expire, or chase them showing you have power enough to fly?