I'm not necessarily suggesting that singers should be "trained" as such but it does seem that less is expected of them in comparison to instrument players.

Most people would expect to put a bit of effort into learning the basics, at least, relating to the mechanics of the instrument and to practise the tune, piece of music, accompaniment or whetever. Or to have developed the skills to pick up a tune or "join in" instinctively.
Of course,this doesn't necessarily mean formal training either. Many of us are self taught.

Perhaps it's because "singing" is perceived as a very natural thing to do that people seem to have the expectation and sense of entitlement to get up and perform or are encouraged to do so by others more so than instrumentalists?

However, a poor performance by a singer is no less unpleasant for the listener. Of course, I'm not arguing that everyone should be perfect but just trying to illustrate a point.