You would guess wrong. Even when DW & I took our first cruise in 2013 on RCI's Oasis, we had the option of "shared table" vs "private" (not really, more in a moment) table in the MDR. We tried the shared table a couple of nights, but since neither one of us is good w/small talk, we went for the private table for the rest of the cruise.
Regarding the private table, there were times our two top was separated from the next two top by about two feet.
When we cruised with family (group of five once, group of six, and group of seven), we always had our own table.
What other lines do, I don't know, I'm just speaking about our experiences on RCI.