I am I suppose what you would call a classically trained singer. I'd never sing until my 30s and started to have singing lessons. I'm a baritone, although I probably should sing tenor). I don't do solo, but I have sung a huge range of the classical repertoire in hundreds of concerts over the last 30+ years.
So what happens when I hear your archetypal Karaoke singer? A great deal of wincing. A lot of the classic American songbook stuff (Gershwin, Cole Porter etc) and the 'crooner' repertoire (Bing, Sinatra etc) was written for competent singers with at least a two octave range. Most (not all) untrained singers cannot cover that range. Which is why it often either varies wildly in pitch or it sounds as if the local feline population is being systematically strangled....
People 'think' that singing is easy. For most people, to sing musically, in tune, in rhythm, and conveying the words and sentiment is hard work.