But to kinda address your point, I think that aggressive teams require more planning then stall.
With stall, you pick only from a select few viable competitive Pokemon with their stall specific ability. Combine those with universal stall moves like substitute, protect, and status moves. Seems that the movesets and pokemon roster are much more streamlined....its more how you use them.
Now for aggressive teams you have to plan out a lot more in that you have a ton more options. Tank, special/physical wall, physical/special sweepers, etc. Way more competitive pokemon to fill the slots, each with their own unique teams.
I dunno, thats how I see it.
Well, if you're talking about Smogon only, then yeah.
Smogon, and all of the things that it is doing and has done, is a fantastic organization for the older Pokemon "fans" to take part in.
Thing is, it's also very insular. Once you get outside the testing and the simulators, online mode "in the wild" is different. Sometimes, this is annoying, but the majority of the time I see really creative teams that break the mold as far as tiers are concerned. The tiers are definitely great guidelines for "how it's gonna be", but in gen IV (the last time I really took Pokemon seriously) I made a pretty great sandstorm stall team, and I think half of it was UU or BL pokemon that were optimized to be tankier or "stallier". I won most of my matches, even when I rule out all of the little kids with "anime teams" and other weirdos. The most boring matches were against the offensive teams, because I could almost always predict what was going to happen, to the point that I predicted probably 60% of my losses (mostly against Metagross, if you care to know).
Also, "stall" doesn't necessarily mean that all you ever do is use stall moves. There are degrees of offensive and defensive traits, and applying them in different ways can yield different things. For example, "tank" pokemon are basically offensive pokemon with "just a pinch" of stall. My old sandstorm team consisted of about 10 interchangeable members, but it only had 3 sweepers, and the rest were essentially tanks who could utilize varying degrees of stall tactics.
When it comes to games like Pokemon (which is almost just a trading card game with a six-card deck), really great players know how to use what's popular (typical offensive teams), but they
also know how to be creative and experiment with things in order to create new strategies that nobody else thought of trying.