Why metals break on dynamic load but not on static load of same magnitude?

That isn't necessarily true. If the metal is under repeated dynamic loading, it becomes a matter of material fatigue and not strength. For instance, if you bend a paper clip back and forth repeatedly, it becomes easy to break. The material properties have changed (more brittle) due to the repeated dynamic load.

In the case of impact or blast loading, it is actually the opposite. Materials often have the ability to withstand much larger loads than they could statically, but for very short durations (on the scale of milliseconds).

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because i said so.

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Dynamic load- Any load which is nonstatic, such as a wind load or a moving live load.

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Static Load - Any load, as on a structure, which does not change in magnitude or position with time