This is a list of military spending by country: https://en.wikipedia.org/wiki/List_of_countries_by_military_...
If you count only France and Germany they spend already more than Russia. The European Union together spend more money in defense that any other country in the world except the USA (and around the same that China).
So, what's the reason for demanding more spending?
Recently, there was a decision to increase spending at the European level, but the project was to spend it in local industry instead of buying the weapons to the Americans. The pentagon reacted really offended (1).
This "you should spend more in defense" is not about defense, it's about money, and where that money goes.
I think a better question that why the Europeans don't spend more in "defense", it's: why the Americans spend so much?
(1) - https://breakingdefense.com/2019/11/pentagon-nato-warn-europ...