There is only war... (why make peace)

Correct.

No default garrisons, so most colonies are defenseless and you can use a rapid assault force to bypass the enemy ships and steal 5 or so planets and just destroy the colonies instantly if you don't think you can hold them...

and while I believe there is a War-Weariness figure in the background telling the AI to eventually make peace if nothing is happening and the war is going on a long time... there is no game-mechanic active on the player. Colonies only "rebel" because of enemy culture as far as I can tell.

No matter how low your morale, planets will never secede/rebel and no matter how many wars you are in, morale/productivity/etc. is never hampered. Your incredible democracy or tyranny enjoys the political support of your people 100% so long as taxes are kept at a good rate... never mind the fact that you might be losing a war to genocidal aliens, or that your peace loving people have been genocidal wiping out alien civilizations... no one cares lol.

Galactic Civ is a pretty great game, I love it to pieces, but it is missing some basic ideas from typical 4X or other Civ style strategy games and how the game treats population management (virtually none) and diplomacy (including culture takeovers/etc.) is pretty weak.

Ah yes, I saw that too. This is a bit annoying, there should be some motivation to make peace, even if one side has the advantage. If there is no motivation, then wars are basically never ended because by definition one side has no interest in making peace.

Can this be modded somehow with events perhaps?

Yes and no. You can add WarTurn Events, however, each event will fire after X (the number you set) turns. They will continue to repeat unless you have set OccursOncePerPlayer, then they only fire once. If you set event, events, chain of events for a supposed "warmonger" this will not force the AI into peace since they will not understand what the events are trying to achieve. They will just see it a as + or - modifier like any other event.

Basicly the event system just is not designed to effect AI diplomacy.

You can't add a diplomatic entry forcefully (peace being one) either I guess? Or you would have told me so

I suggest the central point of blame is that the vast majority of 4X fans favour the Xterminate part of 4X over the other 3 bits.

This allows those folk to kill to their heart's content.

But, yeah, constant war should have a negative effect on civilization morale and your government/ruling party's popularity and other things like productivity, etc. Even if you're winning, there's still metaphorical sons and/or daughters (or young versions of Drengin, Yor and so on, whatever you call 'em) being away from Ma and Pa and dying/risking dying and so on. And especially if you're trying to be Benevolent/Pragmatic, carrying on a war just because shouldn't make you popular.

'Though the Yor would be more worried about lost production, wouldn't they? Since they're just machines and have no feelings for family, etc...