Weaponized insectoid robots are called drones, they’ve already been buzzing around killing people for over a decade.

As for why they aren’t using insectoid/centaur-like frames for infantry models, I’m guessing the training data. There is a decent amount of human motion capture data, and an unfathomable amount of video of humans moving that could be converted into motion tracking data through AI image processing.

As such the AI that runs these robots could be trained on huge amounts of virtual information based on real life scenarios, vastly reducing the time the AI needs to pilot actual robot frames to get any good. That is the primary bottleneck, so it makes sense humanoid robots would appear before centaur ones.

Still, there are undoubtedly parts of the military-industrial complex making centaur robots or different body plans, they’re just not being rolled out in combat scenarios yet.

From the original Time article:

The Phantom MK-1 S looks the part of an Al soldier. Encased in jet black steel with a tinted glass visor, it conjures a visceral dread far beyond what may be evoked by your typical humanoid robot. And on this late February morning, it brandishes assorted high-powered weaponry: a revolver, pistol, shotgun, and replica of an M-16 rifle.
…
In February, two Phantoms were sent to Ukraine—initially for frontline-reconnaissance support. But Foundation is also preparing Phantoms for potential deployment in combat scenarios…

However, the difference between holding a weapon and firing a weapon accurately is an ocean wide.