Would they send me a bill after a night out at the pub?
Wouldn't a pay-per-song scheme destroy independent bands so that only the biggest and most shared get listened to?
Same as when dialup Internet access was billed by the minute. Most people would limit their time online to reduce the bill, and didn't feel free to browse for hours every day if they found something interesting. Email and to some extent Usenet were popular because you only needed to go online for a few minutes to download and upload your messages; you could read and write offline for free. When unmetered "always on" connections arrived people's habits changed radically.