It would be more practical, if everyone spoke the same language, but the EU does deliberately not want that. The EU wants to preserve language and culture of its member states and regions. "United in Diversity" is an important idea of the EU.
One advantage of it being "dead" is that the meanings of terms are much more stable. They don't undergo the usual slippage and mutation of spoken languages. This advantage would be lost if it were to replace existing vernacular languages.
While that might make sense from a legal standpoint, it seems like it makes it risky to hire patent lawyers in Europe who are not native English speakers, which in turn, along with many other similar inconsistencies, does in practice create an EU-preferred language.