I think it sounds silly for them to sing in phony US English that they've learnt from watching TV series. I.e. "I ain't gonna come back no more..." and so on. One of the main points of being European (which is the theme of the competition...) is to have your own national identity. Not to speak in another language and imitate another culture! If it's just a competition about singing a pop song or a ballad in English, then it's not really in the spirit of the original idea.

I suppose singing in English means that about half of the viewers are able to to roughly understand what the song is about. So that is probably the main reason why they do it.

But it's not like the songs usually have any deep or meaningful lyrics....!

A few years ago when I watched it, they tried to interview one of the contestants, who had been singing in English. It turned out that he literally didn't speak a word of English. He had just memorised the entire song.

When I watched it as a child, I am pretty sure that everyone was singing in their own language. But slowly the trend moved and now most are singing in English.

I have to laugh at Sweden were everybody takes Eurovision SOOOOOOO seriously.
I had forgotten about it until I checked a Swedish paper online.... people are obsessed with it.