The reason that, in my opinion, Dungeoneering can easily be a tester for level 120 in all skills is simple: its basically pointless. That doesn't make it a bad skill, it's just to say that there are few exterior uses for training it. The shocking thing that many players don't understand is just how much experience it takes to achieve level 120, since experience between levels is exponential, you would need 104,273,166 XP versus the 13,034,431 XP needed for level 99. This is a mammoth amount of experience, especially in some skills that have slower training rates, such as slayer. In the skill of Slayer, which maxes out at around 30,000(30k) XP per hour, it would take you 3,041.3 hours to gain the marginal 91,238,735 XP needed for level 120. In order to put that into perspective, playing 10 hours per day at maximum efficiency, you would need to play for 305 days (on the 305th day you'd have light work
). Currently only five players have enough experience to qualify for level 120, this means that if Jagex was to release a level 120 slayer monster, they would be billionaires in a matter of days, even if the item required 120 since about 110 players would qualify for 120 attack and about 75 for 120 strength.
Let's look at a skill where experience is more or less capped per day, such as farming. Planting 5 Magic Seeds, 5 Palm Seeds, and 1 Calquat Seeds earns you, assuming nothing dies, 133,353 experience per day. Gaining level 120 from 99, which takes 91,238,735 XP would take you 685 days. It would cost you, at current GE prices(though I'm sure they'd skyrocket once 120 was announced) 2,256,728 gp per day and 1,545,858,680 gp overall. Currently only 11 players would qualify for the 120 level farming range.