Are you guys sure? I see on the wikipedia list that Borg became number 1 on august 23rd 1977 and held that position for just 1 week.
McEnroe also got one week in 1980 - aug 11th. And there were five more "one-weekers" in 1982 and 1983, courtesy of Connors and McEnroe.
They did occasionally release rankings two or more weeks in a row, just not regularly.
I did say 1973. Before 1973, there were no computer rankings, so the uncertainty is high, after it, the number of issues per year increased. And here is how from the ITF site:
They were not weekly as they are now (except for non issues in the middle Slams or during the events in Miami and Indian Wells) until 1979 with there being the following number of issues in the early years (and not always on a Monday).
1973 - 7 issues
1974 - 11 issues
1975 - 13 issues
1976 - 23 issues
1977 - 34 issues
1978 - 40 issues
1979 - 43 issues
So yeah, Borg got one week because there was a moment when two issues were published one week apart. So did McEnroe. Summer #1 moments were possible due to the fact that the season was intense during the summer, so frequent ATP ranking issues were published then