There was actually a really good article in the FT of all places on this subject: https://www.ft.com/content/37053b2b-ccda-4ce3-a25d-f1d0f82e7...
The fact that the FT is picking this up should tell us something given its typical perspective. There are two big groups of countries in this situation concerned with keeping russia in check: America and the Euros. The former has less of a direct concern but more ability to do something about it; the former have more concerns but less ability. So we settled on a compromise where each country would contribute a proportion of GDP rather than a dollar figure. This is fair-ish; it's still a huge benefit to the euros, but pretty fair. Yet for decades, they have consistently failed to meet their proportional obligations, instead directing those funds to things like "free healthcare".
Other major reasons they can do this include not having debt from having to finance the rebuilding of their continent themselves.