Parties are small because it's exponentially harder to find a party of size X+1 than X. WoW has a party size of 5, and tanks are still the limiting factor in their group finder. EQ1 has a party size of 6, and getting the tank is again still the bottleneck. Putting more DPS slots just makes more people play as DPS, since the tank role is either too hard or irrelevent (e.g. the tank on Anantaboga could be a trained monkey) depending on which game you're playing and nobody likes either of those two roles.