The main reasons to go for total war:
1. Truce timers are dynamic + a flat 5 year. This means that getting two 50% WS deals locks you for 20 years while one 100% (which can take more to boot against a big enough opponent) locks for 15. Playing some sort of historical death of a thousand cuts against places like Hungary or India just is less viable.
2. AI peace logic is terrible. When the AI will cough up 20% WS, it will often be just a year and maybe half again more casualties from coughing up everything. If it costs me X amounts of manpower, favors, cash, etc. to earn a booty of 20% WS and it costs me 2X to take 100% WS, total war is two-and-a-half times as efficient as a limited war.
3. The ability of rivals, coalitions, and opportunistic minors to intervene is basically nil. Truces, for the AI, are essentially inviable. Cycling truces means that staying at war for another year or two to carpet siege somebody comes with no risks. Even if the Russian AI ships the whole army to China and somehow Sweden makes an opportunistic attack for Estonia, it is virtually impossible for the AI to force concessions before Russia concludes some peace in China and then turns her full might on Sweden.
4. The long term diplomatic issues favor total war. AE gain for historical conquests is routinely in the range that game ends before it burns off with the target (who will also hate you forever thanks to the cores you took). Killing off a state reduces the number of people who hate your guts forever. Going after religious-cultural blocs sharply decreases the people who want to attack you.
Every measure of efficiency says that total war and getting 100% WS or something close thereto gets the most benefit/cost. External threats are locked by ahistoric truce mechanics and even when somebody makes a classic historical play for limited gains, they have no way to force concessions before they are locked in a total war.
Somehow, somewhere the Devs became convinced that players would just punch out AIs for limited gains. This has lead to multiple interlocking mechanisms that make total war overwhelmingly more efficient. To the degree that the AI does not play for total war, it become worse against the player. All the conquest slowing mechanisms, all the speedbumps to incorporating your gains ... all of these hit the AI far harder and result in the player much more easily outgrowing any individual AI and rapidly any bloc of AIs.
If you want to make limited war viable without implicitly nerfing the AI you need to make limited war more profitable than total war: have ways to force a low loss peace for limited gains, let nations violate truces for lesser penalties when rivals are on the rampage, and above all, have the AI start cooperating more efficiently when some state starts kicking around everyone in the area (e.g. as historically happened when a bunch of rivals all began dogging the Ottomans at the same time).