Because eventually there will be enough changes to the upstream Chromium codebase that the only way to keep these extensions working would be to stop following upstream, which would mean massively increased development costs.
But they don't do adblocking with ublock, so it'll be safe
Chrome is going to continue supporting v2 extensions for enterprise users, so presumably it'll be pretty trivial to keep support for everyone else.
Google says that's only going to last one additional year, not forever.