There was 52x and 54x for CD-R. The main reason such things could appear wa probably because the Taiwanese media manufacturers were willing to introduce such speeds in cooperation with the local Taiwanese drive manufacturers.
With DVD+R and DVD-R, things have changed back. Taiwanese media manufacturers and drive manufacturers had problems keeping with Mitsubishi, Hitachi, Toshiba, Sony, Sanyo, Ricoh, Pioneer, and other leading manufacturers. Since the largest maker Ritek had problems even at speeds like 4x and 8x with DVD, they couldn't think of introducing anything higher than 16x.
For marketing, it would work. Afreey once announced they'd release 25x DVD-ROM drive working with Zen which never appeared. Hm... I'd like to have a 18x SCSI DVD-ROM drive.