Well, it depends on how important you think vocals are. For me personally they're just one instrument, a part of the whole, and of course lots of tracks don't use vocals much or at all; I also tend to prefer studio versions as the default version of a song, and these of course use the best possible recording (even leaving aside possibilities like pitch correction). So for me, singling out vocals of a live performance is is a fairly minor thing and if the rest of the music was also live, I doubt people would also pay this close attention to, say, whether the percussion sounds perfect or not.
Keep in mind I'm not necessarily talking about cases where the singing is blatantly and objectively bad; more often than not it's nitpicking about small details and people without experience acting like they're vocal coaches. I am aware though that there's a common tendency to treat the singer as the artist (even if the song is completely written, recorded and produced by other people and the singer's only input is the vocal) and singing as the most fundamental part of a song. I guess the most visible person essentially becomes the "face" of the song (similarly how singers of rock bands are generally seen as the frontmen).
I just think that in ESC, we end up in a silly situation when instead of (both the audience and juries) critiquing the song/composition as a whole, thanks to a) Eurovision placing the most importance on the live performance and b) only vocals being actually allowed live, it leads to singing getting disproportionate attention.
