I think it's mostly an accident of design. For the arcane casters they opted to provide full casting gish options with multiple attacks instead of hybrid class designs. Warlocks, Wizards, and Bards all have subclasses that provide 2 attacks and full spellcasting. There's little need for martial casting hybrids. I would be open to a swordmage that combines casting and fighting better than a bladesinger or eldritch knight, but there's no real need for it. The spellcasting warrior is pretty well covered.