When you sit on a frame you apply a load. When you get off you release the load (more load for some of us). The frame is stressed with some slight deformation and then is unstressed and (hopefully) returns to its first shape. When I supervised an undergraduate materials science laboratory one of the experiments involved loading and unloading a material until it failed. As I recall aluminum is not as "flexible" as steel. Steel will deform quite a bit before failing but aluminum tends to bend and "snap".
I have a "small"-tube-diameter aluminum Vitus frame. My impression in riding it (I don't ride it often because I am too heavy for it) is that it is very flexible. In fact, in a hard acceleration it reminds me of a wet noodle (slight exaggeration). This frame, when I put my load on it flexes quite a bit causing fatigue (to the bike frame) and if I rode it a lot it would likely fail (crack and break) at one of the lugs. If one makes the tube a larger diameter then it becomes "stiffer" and the amount of deflection is reduced and the frame lasts longer under load. I suspect to get the same sort of product life out of aluminum tubes that you get out of steel you have to make them a bit stiffer than a steel tube would be.
That's why we don't make springs out of aluminum. The sort of load cycles that most of us put on a steel frame do not exceed the material's ability to flex and return to shape, nor is our weight generally high enough to permanently deform the steel frame. One would expect a steel frame to gradually bend and then finally "fail" (unless it had been otherwise damaged). With aluminum one is a bit closer to the material's limit.
My son's aluminum fat tube Trek 2000 seems quite strong and light, but also a bit stiff compared to a steel frame. It also has a steel fork -- (I think Vitus was the only company that made a "good" aluminum fork) -- one wants a bit of a "spring" up front. One the other hand I think the Trek weighs a bit over 21 lbs and the Vitus is close to 18. My steel bikes tend to weigh in at 23-25 lbs. For those of us who don't worry about weight as much as comfort a good steel frame is hard to beat. IMHO