The overall standard is usually slightly higher than the World Cup, so that often sets it apart, but I think the crowds play a big part.
Each nation’s fans usually only get 10-15k tickets for each match in the World Cup, with the rest going to sponsors, FIFA, and corporate packages.
Whereas 90% of the tickets for each Euros match goes to fans of the nations playing.
Creates a completely different atmosphere.
It also helps that most of the host countries are already setup to put on big tournaments, and are within easy travel distance of the vast majority of participating nations, so although there have been transport and access issues thus far, it has still been far better than Qatar or Brazil (or South Africa) for fans and teams alike.
And that makes a big difference to the quality of play on the pitch.