Personally I think it's because the AI became so much more passive and preservative (they're almost no longer willing to sacrifice their unit to kill mine, except for barb), plus the changes to city defense that makes waging war no longer profitable and they rather just sit tight and build up.I'm trying to put my finger on exactly what has changed, but in general I'm noticing the AI just not as good at war as it was say 6-8 months ago. I'm steam rolling over Immortal AIs now, when back in the day these wars used to be a serious endeavor for me. Again, I'm not exactly sure what has changed, I'm trying to suss it out as I play, but the just feels more timid, and maybe isn't making as many units. So not the most productive post, its all I got at the moment.
Honestly I would have much more fun with aggressive AI in some old VP versions, since combat is what makes player shine and improve player's experience (the next best thing is moving things diplomatically to lead to war/combat), rather than playing builder all games. There's always more buzz about "hey I succeed defending/capturing this city" than "hey I built a building/researched a tech/got a policy/..."