I don't think we are slow in adopting parallel charging, simply that for this application until higher current output chargers were available there was little to no advantage in doing so.

Until recently most of the chargers were limited to 7-10A, but could do 10-14 cells in series, so it made more sense to series charge. For instance, the TP1010 was probably the most used charger, for a 10s 5000 mAh pack (x2 5s 5000), the time to charge in parallel or series is the same, but it is more work to make the pack ready to charge in parallel.

Likely with some advantage to be had, people will start parallel charging more.