I don't think it's pointless. A large number (the majority?) of users probably don't need install scripts, so disabling them by default is a net security improvement. Those that do can enable the insecure behavior, which will become an explicit decision that is trackable, auditable, etc.

You're not wrong about sandboxing, but sandboxing isn't something that can just be blithely introduced to a large packaging ecosystem that previously assumed full system access. Doing so results in the same kind of regression you point out: if the sandboxing breaks peoples' builds, they'll just disable it and move on with their goals.