Title. There are enough unfair things to contend with besides this. A solo run can essentially fall apart for no reason at any time, at no fault of the player.
Upon disconnecting a player is removed "physically" from the instance which I imagine they might be fearful could lead to cheating by making monsters disengage? However there's a good period where our characters remain in the game before actually disappearing, as we know from other instances, even when alone. DC'ing after the final boss of a dungeon, you can log back in to still be the only person there.
To be honest, in the brief period before reconnecting there's a good chance on later floors that we're probably gonna be dead because of the inability to act anyway, but at least there would be some chance of recovering if we can get back in - and it would make players who love the solo challenge like myself feel significantly less helplessly-frustrated, and like we still have a degree of control when the unfortunate 9002 or router hiccup happens.