It's worth noting that Saber's the strongest class, but the strongest Servants are both	Archers. Classes are just a guideline.

Saber probably works out as the strongest because of just how many heroes that slot into the classes are primarily known for swords, and the world's glut of magic swords. There's no need to assume it's an intentional conspiracy. The knight classes are the best in combat because [i]that's literally the qualification[/i]. Berserker's also good at that but if in full-crazy mode disadvantaged against smart opponents (it's not like strong Berserkers can't overwhelm their opposition). Caster and Assassin are by default [i]not[/i] combatants.