Some game have "lost" ownership. Say that old game studio that developed GameBoy game has gone bankrupt, no one is going to enforce the copyright

Most game or not reedited so there is no money to gain fom preventing them from being online. It may even help to keep the brand alive in case a reboot in programmed (Eg: tomb raider)

Often it's a simple calculation of how much revenue could be won if you factor in the fact that you need to pay the team to find the ROM's, send the letters and enforce them once in a while. Basically it's nearly never worth it.