Even with the same writer you don’t have to tie your universe together as tightly as Marvel always had. It’s easier to do with limited writers because they know what they wrote, but you can still do it. Where you can really see how much DC lives in silos is Batman. At one point Batman was in 5 books: World’s Greatest, JLA, Outsiders, and of course “his” books, Detective Comics and Batman. But those universes barely ever touched each other, Outsiders ostensibly started because he left JLA, but he went back and it was never mentioned. Heck even the death of Jason Todd in Batman barely made a ripple outside of that book, it took them almost a year to even MENTION it in Detective (admittedly Detective always leaned to Batman without Robin, but a YEAR). They just never liked pollinating one series with events in others, outside of a few mega events like Crisis or No Man’s Land.
One of the big things Marvel always did was leverage that shared universe to push sales. It’s always been telling to me that the Avengers was usually built around B characters as opposed to JLA being built around the A’s. If you’re an Iron Man fan you can’t just get Iron Man, you have to get Avengers or you’re missing a bunch of his story, and now you’re getting the Avengers which is being effected constantly by events in Cap’s title and The Hulk etc etc. In relatively short order you’re buying half the Marvel titles just to know what’s going on. In DC if you were a Batman fan you never needed to get JLA (I know, I didn’t) because JLA functionally was its own universe. Even today with the New 52 DC keeps things very isolated. They’ve broadened a little bit, especially in the Batman world where they have Bat Inc and all of his former Robins have their own line and they interact a little bit, but you can still pick and choose in DC and not really miss things.
It was Stan’s way of writing. He later moved up to editor then publisher and it became Marvels “house style”.
DC never had one person over story direction. Batman and Superman have always had different editorial direction and virtually separate universe’s. Heck, DC pretty much had the same staff from the beginning of comics through the silver age.
It was DC house style to have no to limited continuity until the Bronze age. Tight continuity was Stan Lee thing. DC, as a company, believed tight continuity impeded good stories. This is still true to a large degree.