Because 8, 9, and 10-speed cassettes are all about the same width. Shimano achieved the extra speeds in 9- and 10-speed by making all the cogs and spacers a little thinner.

For example, a 7-speed Shimano cassette is about 32mm wide, and the hub has a little bit less than that to ensure the lockring can compress it adequately. If you were to do an 8 of 9 on 7, you'd start with a 9-speed cassette (36.5mm), remove one cog and spacer (4.34mm), and end up at 32.16mm again.