The big thing is that most of the competitor states, like France or Austria, cannot grow at the same rate. Up until the Reformation all the Catholics bar Sweden, Hungary, and Poland/Lithuania can really only gain land by ticking off fellow Catholics. Worse the HRE has much more AE. So Austria, France, etc. spend a lot of time waiting out the AE that comes from much smaller land grabs. The OE, in contrast, can beat up Serbia (Orthodox), The Mamelukes (Sunni), QQ (Shi'ite), and Venice (Catholic). This makes them wildly more able to out expand France and the rest and is made worse by their age ability for sieging (at a time when very few states have siege bonuses of any sort).
Then the AI has trouble adequately valuing troop quality against numbers. Having a giant pack of manpower deters a lot of AIs from attacking or even forming a coalition. And lastly when the OE does expand towards the states that historically defeated it (Russia, Poland, Austria, Venice, and the Pope), far too often they are rivals or otherwise hate each other and gift the OE more room under the AE cap before the coalition forms.
Then should they finally face a war with another major AI or bloc thereof ... the OE terrain is terrible for AI invasions. Some AIs will march around the Black Sea. Some will go after the border forts in the Balkans, and some my head into the general vicinity of the main army. The Ottomans, in contrast can walk across the straits and do a lot of defeat in detail. And when the OE has pathing troubles, it just cannot get as badly lost (outside of Russia) on the way to Wien or wherever.
That said, the OE is fragile. If they don't get rolling quickly they can get punched out from 4 sides. Being on the crossroads of religion with lots of potential expansion means that the OE is a lot more land to grab below an AE cap for the folks who border it. If Venice's League crimps the early game, there are a lot of places that want a piece.
And then we run into the same problems. When I have a micro-OE I almost inevitably have somebody huge in the Balkans (e.g. Hungary) and somebody huge on the Eastern or Southern borders. And then we get back to a lot of the same dynamics - too few AIs cooperating too poorly to take down the big bad empire.
I agree with all of this, and it makes sense.
What I mentioned quite early on in this thread that for sure if the Ottomans had ended up in Northern Italy, on the borders of France, like they had done in my other save, I am certain there would have been some Catholic intervention against the Ottomans. Someone mentioned this before, but a kind of "We don't like each other, but we'll defend each other against X nation" mechanism would be useful here.
But yes it's true, I've never seen one power cut down to size by another power in EU4. Whoever starts growing in the early 1500s is usually who ends up the biggest by the end of the game unless the player does something.
And as I mentioned, all their forcelimit buffs + them taking quantity means they have an army so large that mathematically the AI seems to look at it and go "nah".
You know, I think a big part of the balance problem surrounding the Ottomans llies in Austria. Austria should (historically) get the PU over Bohemia and gain at least a part of Hungary, but in the game I barely see this happening. This has multiple negative consequences. Hungary get most often divided by Bohemia Poland and Otto, and the Otto's can than push through towards Prague or Vienna. Bohemia or Austria are often emperor and rivalling each other, thus the emperor will often not join against the otto's but stab the other in the back.
Austria at the moment is just too underpowered to offer a balance against Otto's or France for the matter. A small buff would be to increase the chance of Bohemia PU happening.
Of course two easier ways to fix that would be a counter to big powers or some better logistics in the game (Otto's shouldn't be able to reach Vienna or Persia without suffering), but one can only dream about mechanics like this
I've found that if they are going to lose early on, it's because they end up in a war with two out of the following countries at the same time: Venice (if they attack Albania), Mamluks, Austria/Hungary/ and Poland/Lithuania.
The snowballing occurs when they focus on easy targets, Byzantium, Turkish minors, and then pick off larger countries individually, Hungary after they've been weakened by revolts, Venice who they've already weakened after beating Albania, or the Mamluks. If they are in 2 major wars at the same time or in quick succession, they get overwhelmed. But in my experience this doesn't happen often, I'd say in 80-90% of my saves they snowball crazily.
Again, I've thought about allying their neighbours in a bid to stop their expansion but say as France, early game, especially with all the vassals, it gets costly in diplo points and I don't really want to wreck my manpower fighting the Ottomans at that stage.
The only real way to wreck them early is to no CB Byzantium, which works if you are a Mediterranean power (Castile, Aragon, Venice, even Austria) and either 1) Wait for the Ottomans to DoW Byzantium, vassalise Byzantium and then call in your allies or just wait for them to attack. All your allies will join definitely because it's now a defensive war and if they don't get Constantinople I think they don't get the "Empire" rank meaning they'll never be as strong as they would otherwise.