Why are the AI players so easy on hard?
Several reasons. For example, the AI is heavily playtested under and customized to the default ruleset and server settings. Although there are several provisions in the code to adapt to changing rules, playing under different conditions is quite a handicap for it. Though mostly the AI simply doesn’t have a good, all encompassing strategy besides “eliminate nation x”. For further details, see AI. To make the game harder, you could try putting some or all of the AI into a team. This will ensure that they will waste no time and resources negotiating with each other and spend them trying to eliminate you. They will also help each other by trading techs. You can use the team command to set teams before the game starts. For AI teams you have to create the AI players first using the create command.
