I liked road biking for cardio when I lived in places that were conducive to road biking for cardio (Boulder and Golden, CO).
I now live in a place where only 10% or 15% of roads in my county are paved. There are large stretches of the year (right now in particular) where MTB trails and trail running trails are still snow covered, but the skiing isn't great. I can get out on my gravel bike during this time of year.
There are lots of cool places that I can ride on my gravel bike starting from my house that I would not be able to get to on a mountain bike (too much rolling resistance, not enough speed to cover the ground I need to cover) and that I can't get to on a road bike (gravel too deep/chunky, roads/ trails too technical).
That's why I gravel bike- I can launch adventures from my house to remote and beautiful places. Lack of traffic is also nice. There are some days that I've been out where I haven't seen a single car in 40+ miles.
These are all rides from my house-
Attachment 458556
Attachment 458557
Attachment 458558
Attachment 458559
Attachment 458560
Attachment 458562
Attachment 458563
Attachment 458564
Attachment 458565
Maybe there's not enough adrenaline in it for some people, but I really enjoy it.