Even in the 1980s, PLCC sockets were available, and PLCC/SOJ was already being used in SIMMs.
Most important is that ZIP offered higher density while still being thru-hole, thus compatible with existing manufacturing technology.
Sockets are generally avoided as sockets are a cost factor. They make only sense for
- (comparably) high cost components, or
- components with high failure rate, or
- components only populated as options
#1 is the main reason that sockets were avoided for average, mass produced devices.
#3 is also the reason why the 80287 was offered as DIP 40 instead of PGA - an FPU was considered optional way into the 1990s.
Likewise options were the only area where PLCC sockets were somewhat common, for example for RAM expansion with graphic cards during the mid 1990s.
While PGA sockets were more expensive than DIP, thy were still lower priced than PLCC sockets, due being essentially the same socket technology - just more pins. The same consideration is true for ZIP. Here as well it was only a different plastic carrier holding the same pin elements.
Equally important PLCC only makes sense, as a bridge technology between classic pinned and SOJ, if a SMD manufacturing process is used. But SMD was still in infant stage in 1980 - it wasn't until the 1990s that SMD became a common place - not at least due many components not being available as SMD.
Using sockets was a way to stay with a single board design, but still delivering variants with different memory population, here 512 or 1024 KiB. This wasn't so much meant for user side upgrade, but the ability to deliver two different memory sizes while producing only one board and add the second set of chips depending on incoming orders. A great idea during production ram up, as it would only need the minimum amount of RAM by default while keeping flexibility. This is especially underlined by RAM being, at that time, the most expensive part, thus minimizing initial investment has a high effect on ROI.
So everything comes down to a rather short window of opportunity in the mid 1990s for mass usage of PLCC sockets in large scale production.
Last but not least, While PLCC was first introduced ca.1977/78 by TI, it did only receive a JEDEC Standard in 1984, which marked the time other manufacturers started to adopt that packaging as well.
ZIP offered higher packing density while still staying with existing manufacturing process.