The intention behind lockdown mode is protection for a select few groups of people such as journalists, that are at risk of having software like Pegasus used against them. It’s to reduce the attack surface. The average user wouldn’t want most of it as a default setting, for example: almost no message attachments allowed, no FaceTime calls from people you haven’t called and safari is kneecapped. Making this a default setting for most people is unrealistic and also probably won’t help their cybersecurity as they wouldn’t be targeted anyway.

A "reduced attack surface" can also be a reduced surface for telemetry, data collection, surveillance and advertising services, thereby directly or indirectly causing a reduction in Apple revenues

Perhaps this could be a factor in why it's not a default setting