Ceasefire and peace agreements also follow a 19th and 20th century logic of war really being politics and diplomacy by different means. It used to go something like this: you fight a war, you get a winning position where it's more costly and dangerous to the enemy to keep fighting than to relent somewhat, you declare a ceasefire, extract concessions, and things go back to a tense but peaceful "normal". Notice how this mostly happened in wars between peer nation states; colonial wars and invasions didn't really sign treaties until the colonial power decided their position was absolute and they needed to get on with the extraction part ASAP.

Most wars in the 21st century have been wars of anhilation/survival and imperialist expansion, where each part sees the other as a crucial threat to their continued survival, and this logic doesn't work as well. The tools and doctrine are still the same, because most leaders and institutions still use those norms from the last century, so they're stuck using the same tools that don't really work now.