Both of those are part of the overland content and are not instanced (except as part of zone instancing in general).
If ZOS wanted to instance them, they'd have to close off the surrounding areas and separate them from the zones, which very much changes the idea of dolmens as world events that everyone can see going on and run to help and the idea of World Bosses that you can stumble across - you laugh, but I crossed a river in Malabal Tor for a columbine, not looking where I was going, ran smack into a world boss camp and got rekt.
There are usually plenty of out of the way World Bosses to solo. Certain zones have pretty isolated dolmens to solo, but if you really want to solo a dolmen, go to Cyrodiil. There are rarely allies around + you might get ganked.
The best hard mode overland content that isn't World Bosses I've seen is Craglorn group quests and Imperial City Quests.
I could probably complete all of them easily with a group of four, have a challenging but hard time with a partner, and found parts of them very hard to solo but doable.
With that in mind, they both have some issues.
Imperial City is a PVP zone, so you have to be willing to do a little PVP or accept dying as the cost of adventure.
Craglorn used to be a group-only zone, but by and large the player base is simply not interested in a group-only zone. So Murkmire was scrapped and if its every released it will be like the current Craglorn.
I've noticed Craglorn has mini booses that are unmarked with a good amount of HP. I ran into one and got it to 60% on my farming toon. Cyrodiil dolmens are meh imo, they seem easier that regular zone ones, which arr still easy.
i always thought they should scale dolmens to the amount of people doing them so mobs are not instantly dead and bots could not survive them
They actually do scale. You get a lot more mobs with four people present than you do soloing. Its just that dolmen groups like the Alikr where you have at least 24 people on the dolmen sort of hits the upper limit on dolmen scaling so the dolmen doesn't break the server.
As for bots, there's one anti-bot feature already present at the dolmens. Players and bots that stand still get an AOE that builds on them and several of them can kill a player. Again, bots tend to go to the very populated dolmens where they are protected by the sheer number of players fighting at the dolmen and they get regular exp from the groups cycling through the dolmens.
So if you want mobs that aren't instantly dead, don't go to the zones with a dolmen farming group (Alikr and Auridon for the most part) and you'll have plenty of Dolmen action with very few bots.