Because there's more money to be made sounding American.
Irish & Brits watch a whole lotta American movies and tv, as well as listen to our music. So it's easier for them to emulate. It's been said singing flattens accents, "But when IIIIIIIIIrish Eyes are smiling" says otherwise. Heck, even American pop 45s from back in the early days had a different accent than today. Also Country singers has no problem retaining their accent, albiet somewhat flattened with superstars going for crossover appeal.
The song subjects also. Since when do Brits use dollars? Unless one wants to sound like a real Manch Indie Rocker, you'd sooner hear lyrics about NYC than Manchester or London.