Security and usability is always a compromise. Otherwise we'd all use one-time pads for everything on the internet.
The public will soon find out that we should.
It's one of the situations where registry lock is a useful tool with appropriate trade offs. It's far from the only one.
Why? It's about threat models.
I would say because it's a small cost for the domain owner in return for protecting against what's potentially a pretty big emotional (and possibly reputational, financial) downside.