The reason is the network effect of enough people having their profiles in one place for genealogy discovery to work as intended. As OP says, he has some relatives in the 23andMe network, but fewer now that people started deleting their data.
Arguably making potential matches less likely could be seen as a privacy improvement for his relatives.
It suggests you your genetic relatives as they join 23andme. There are genetic reports that are added or updated wi5 new discoveries in genetics. Both reasons are quite minor benefits and i wonder if 23andme will continue at all.
(Disclaimer: I never used 23 and me, so this is entirely hypothetical for me.)