Actually you have the same holiday but you're arriving at your destination in the morning after sleeping all the way, instead of waking up early to be at the airport 2 hours before the flight etc.

Sleeper trains go at night generally.

This, and also for many of us riding the train through picturesque European countryside is part and parcel of taking a holiday, though of course ymmv.

I mean, for me taking a holiday is time to slow down and enjoy the sights. I miss a hell of a lot of that when I fly.