It's not unusual for publishers themselves to be hazy regarding exactly what rights they have retained or sold through the years, and these issues only get murkier as time goes by. Things are further complicated – often intentionally so – by companies who acquire IP in order to ‘sit on’ it with a view to taking legal action should a confused party green-light a project blindly assuming they have the rights tied up.
This article does a good job of explaining why not all games from the past have been made available on modern platforms. As shown in the quote above, there are issues with companies understanding exactly what they are legally allowed to do.
But even once they green-light a project, it’s more than just running the game on an emulator and shipping it out the door. They need to pay devs to make that happen, they need to pay QA people to make sure everything works like it should, and they need to pay people to market the game and create art for the store listings…
I wish this wasn’t this complicated, but at least we have emulation.