I am going to respond to this thread from a 3.5 perspective, since I am unsure as to which edition you're getting information from, and 3.5 is the edition I am most familiar with.
Please understand, according to D&D, "Angels" and "Demons" are much more varied and complex then your OP seems to suggest.
First, on Solars and Planetars:
According to the text in the 3.5 MMI, Solar reads, in part, "Solars are the greatest of the angels...only the most powerful of fiends approach their power." With a challenge rating of 23, that's saying something.
Planetar entry reads, in part, "Planetars serve as mighty generals of celestial armies" at CR 16, that's nothing to sneeze at.
These guys are not weak.
Nothing in these entrys restricts the number of these beings, and they are described in plurality.
Second, what you've labeled "Angels" are actually part of the large grouping of "Celestials".
MM1 Angels- Astral Deva, Solar, Planetar. Any good alignment.
MM1
Guardinals- Avoral, Lenoal. Neutral Good.
MM1 Archons- Lantern, Hound, Trumpet. Lawful Good.
MM1
Eladrin- Bralani, Ghaele. Chaotic Good.
These are all the "Good" guys in the Outer Planes that represent anything "Angel" like, and these are only the examples from the first Monster Manual, not including many other official sources.
These Celestials are spread across multiple Planes of existence and are not united in location, organization, or even alignment views. There is no indication that the actively disagree or wage war with each other, but also no indication that they band together very often for any purpose. Their levels of power varies widely between the Lantern Archon's CR 2 and the Solar's CR 23. "Weakness" and "Strength" varies greatly.
Now, on the side of the "Bad" guys. You've got the main distinction between Chaotic Evil Demons and Lawful Evil Devils, and the Blood War between them. First and foremost, it is this Blood War between the evils that keeps them from overrunning the Multiverse, the bad guys are too busy fighting each other though outright combat and manipulative scheming and backstabbing to do too much damage to the rest of everyone else.
Devils vary in species and power. Demons vary in species and power. They exist across multiple Planes, and layers upon those Planes, and are constantly warring among themselves. Demons fight with Demons and Devils fight with Devils in addition to the larger war they have.
The reason why Demegorgon hasn't yet rushed the Material Plane is not a fear of the Celestial's retaliation, but because of political maneuvering and ongoing war. If a single Demon Lord tried something that grand, any number of rival Demon Lords, Celestial champions, mortal heroes, etc. would try to undermine his attempts, both covertly and overtly.
Conflicts on minor or major scales such as these is why we have adventures to play in this game! Any DM
could run a campaign where a Demon Lord tried to rise up and attack the Material Plane, but that DM would have access to just as much powerful options on the other side to design a balanced opposition. The game creators saw that we all had enough options to choose from.