I think that back in the old days, we really didn't think about heroes needing to fit in their solo stories with their JLA stories.
With only one JLA issue a month, readers were able to fit the heroes in logically. Batman and Superman appeared in about four titles each a month, but we still managed to hand wave it and not care. They just had to go to the other dimension and fight the multi-dimensional tyrant in-between their solo books.
Of course, continuity wasn't so tight back then. Stories with beginnings and endings could be slotted wherever a reader wanted, as long as appearances lined up.
Today, it would be harder because issue x leads into issue y, and issue y leads into issue z.
When you have four titles that don't have clear breaks between stories, where would they have the time to go on a two issue jaunt into space or other dimensions?