Yeah I'm surprised the day splits aren't out yet, you wouldn't get many festivals that do it this way, but as you say there's probably some boring reason potentially.
As for the reasoning about day tickets, think of it like selling a multipack of crisps vs selling single bags - yes, 6 singles bags = more profit than the multipack, but they will sell more and ultimately make more money that way, unless they get very lucky in having 3 equally strong day lineups that could sell out individually. Also, there's probably some stats around weekend goers spending more time on site, potentially some promises with the local council about economic spend in the city, and also for the buyer, a weekend ticket gets you access to the stuff either side of the 3 days.
Anyway, this is my first post regarding this year's fest and I'm PUMPED!