We currently all (most) learn English in school, but it's a language with many exceptions, not clear pronunciation and other issues. Even after several school years, a lot of Europeans can't speak English (just one source, but there are many others) https://historiccafesroute.com/wp-content/uploads/2022/07/conversation-english-eurobarometer.jpg
Esperanto is a language that has been intentionally created to be easy to learn. It has no grammar exceptions, pronunciation is straightforward, and words are easier to remember than in English https://en.wikipedia.org/wiki/Esperanto#Vocabulary
There was a study that I read some time ago about a test project in Finland and Hungary, and it concluded that students become fluent in Esperanto much faster than English. So a lot of time and efforts could be saved for other things, and a greater percentage of people would become fluent in this common language, than currently with English.
It doesn't have to be Esperanto, it can be another language that is intentionally easy to learn. But the idea is to have a language that is both easy and faster to learn than English currently. More people would learn it, and compared to English, a lot of time could be saved for other things
I don't get why this isn't done, wouldn't most Europeans benefit from it?