you are an outlier - most Blink users use cloud, most Blink users do not insert SD cards, most Blink users power cameras by batteries. Those users have no use for continuous recording.

Could Blink implement it? they sure could. Would the company benefit from this? I am not sure. On the upside, a very small fraction would be happy. On the downside, this is more code to maintain. Plus, regular users who turn this option without understanding what it does will complain (to support and forums) about battery life.

This is where the open-source firmware can be handy BTW.