yeah. the reason is that they can get away with that.

this change was bundled with another one that was kind of good to have: building apps to an .aab file and making split apk's out of it.
but in this scheme the dev builds the .aab, and google makes the split apk's, and google needs your signing key to make the signed split APKs. the reason they need your formerly used signature's keys is because if they would have started signing apps with a new one, users who had your app already installed would have had to first uninstall the app and lose their data, because android has a security feature that does not allow an update that has a different signature.

of course, while at first it was an option, the play store has soon made it a requirement that you upload your apps as .aab files.
developers basically didn't have a choice, other than not releasing any more updates to the play store and letting google delete "outdated" apps when they want, like they'll have a sweep soon.