Although a Lich can be reborn over and over, few Liches detonate their own bodies when they haven't been driven to desperation.
The body of a Lich is created by the constant accumulation of dark magic power, and it is itself a high-order Undead. The magic power that Amberser requires for casting spells is saved up bit by bit over time.
A body like that is almost as precious as the Life Casket.
At any time, having the body destroyed is a heavy blow to a Lich, which is why no one expected Amberser to self-destruct on the spot.
The dark magic power that had been refined to its utmost limit erupted in an instant, forming a pitch-black tide that swept across all directions, enveloping all the Magipuppet Automatons that had been summoned.