Its because SV doesnt know how to balance them. They made it so that you can raise as many walkers as your mana allows, which is crazy OP if you think about it. Running around with 6 or 7 or 8 greater walkers is right down busted. So then, in order to make it in any way fair, they made them incredibly frustrating to control (have to cast control on each one of them) AND made them completely retarded and slow.
They should have just made them so that each additional walker halves the strenght of all your walkers. That way you have to choose between 1 strong walker or many weak ones. Also remove that “control walker” garbage mechanic which makes raising walkers during pvp basically impossible. You need 5 minutes prep time to summon and control all your walkers.
Instead SV decided to make them so problematic, cumbersome and impractical that nobody uses them.