This is a very old accusation. In fact it's very rarely deliberate. Without going into a uge long history. American Country music is largely influenced and drerived from trad Irish ballads. And of course many American settlers were irish or of Irish descent. Now irish people pronounce their R's very strongly as do most Americans for the obvious reasons above. Hence much Irish music and country songs led to an accent heavy on the R's. cut a long story short most song in particular Pop has evolved from Country and you get most singing accents derived from these rather American/Irish lyrics. I've heard Folk sang with thick Brummie accents take it from me it sounds dreadful