It's not that there can't be both offensive and defensive Pokemon on a team. It's just that Pokemon of conflicting play-styles do not synergize well together. It really has to do with momentum and the objective of the team. Stall is meant to sit there and be fat and rack up residual damage until the opposing Pokemon faint (or the player rage quits). Stall teams have no momentum. Balance and bulky offense focuses on utilizing both offensive and defensive Pokemon to either deal damage or take damage, respectively. The objective of these teams is to put yourself in an advantageous situation by outplaying the opponent. To do this, balance and bulky offense need some momentum and thus appreciate VoltTurn cores. The type of defensive Pokemon on these types of teams can soak up hits, but they don't sit there are do nothing. Rotom-W can spread status and Volt Switch out, defensive Lando-T can set up Stealth Rocks, Knock Off items and U-Turn, Magic Guard Clef can set up Stealth Rocks, spread status and set up... you get the point. What does Mega Sableye do besides sit there and Recover stall? (usually the Calm Mind variant is not seen on stall.) The answer is nothing, so it loses momentum and puts Pokemon like Landorus and Heatran in non-advantageous situations. Same with Unaware Clef- its meant to soak up damage and stop set up sweepers, thus decimating the opponent's momentum. There's a lot more to it, but this is the premise of how Pokemon thrive in different playstyles based on the objective and moment of those playstyles. And so, there is ambivalence (love the vocab Muguet).
