They're catering to their fanbase. Most longtime players never bothered with the campaigns at ArmA 1 or ArmA 2. ArmA 3 SP is the mission editor. As it was in ArmA 2 and 1..
They were likely shit anyhow. I don't know. I never touched them and I've had OFP and ArmA installed on every machine I've owned in the past 13 years. I feel pity for any gamer here that actually tried to play through a post 2005 BIS campaign. You guys are true soldiers.
Why they would choose to set it on a weird Greek island and then not include the campaign that would no doubt have given the Greek setting a point - that is truly beyond me. I can only imagine the terrible, terrible campaign they had planned. World War 3 is triggered by elicit trading in olives?
They should be choosing themes. Cold War. Jungle. China versus America. BIS are out of touch and incapable of producing anything with broad appeal. The only reason OFP's campaigns were decent was because Codemasters whipped the Czechs.
They're not capable of developing their own engine into an actual "game". I doubt any of them even play it.