They haven't. They genuinely don't even know how much it'll cost to rebuild or what it looks like. They EU and media purposefully lowball or don't report the public figures of reconstruction costs to not put people off Ukraine.
Keep in mind there Russia controls Ukraine's industrial heartland, 90% of its energy resources (including all of its offshore oil), and its critical ports and shipping. 70% of Ukraine's Exports (35billion yearly) have to go through the Black Sea. It's already highly likely that Ukraine will lose some of its production capacity permanently – e.g. just the destruction of Mariupol and its two large steel and metal work plants will mean a permanent loss of about a third of metals output. The Ukrainian GDP will contract by 35-50%, and the forecast recovery path for Ukraine is far shallower since recovery tends to be much harder in industrial or service-based supply chains, and the impact on capital and total factor productivity is therefore going to be far larger.
There won't be a 15% GDP growth or a Ukrainian miracle as soon as the conflict freezes, It'll be incremental 2-3% GDP Growth, see here.
https://www.fitchsolutions.com/coun...d-2023-no-return-pre-war-gdp-sight-10-10-2022
In 2021 Ukraine was already the fastest shrinking society in the world and probably the first nation in history outside of war where there were 3 times more deaths than births. The demographic situation is way worse in Ukraine.