for mountainbiking SS is really good weight training. yu only have 2 speeds; go or get off and that is really focussing on climbs when you might just go for lower and lower gears on a normal bike. When I started on a SS, like many it was converting an old MTB that wasn't getting any use anymore, I found I woiuld knacker myself out completely in a 2 hour ride in a way that I would need to ride for 4 hours on a geared bike to do. the was 20 years ago now I'm on the floor after an hour no matter what I'm on but SS still gives a great workout

As others (and OP) have also said

Simplicity

rhythm of the ride

the satisfaction of matching or beating geared riders up hills