Because if that were the case then ship based battles would be over very fast indeed..
And indeed they are. Shields block phasers; if there are no shields, the hull will go away, as in ST2 where Khan cuts into the
Enterprise and Kirk into the
Reliant.
The behavior of the narrow phaser beam here is similar to that of high-setting hand phaser beams applied against walls: it creates a tunnel of emptiness in the target, rather than spreading the go-away effect from the impact point to the entire target. It's probably a mixture of parameters the user can select and physical laws that govern the propagation of the disappearing effect in various materials. Or whatnot.
Now, good phaser hits in the older shows quite often made ships "gone" altogether, although this generally involved a big fireball and implied that the narrow hole made by the phaser had reached something vulnerable. Modern ships just get Swiss-cheesed, such as in the DS9 battles.
Then there's the ability of the
Defiant's "ablative armor" to resist phaser blasts to some degree. But the very name suggests that the armor indeed goes away when hit! It would be rather easy to imagine a material that welcomes the disappearance effect of a phaser beam and allows it to propagate - all of a tenth of an inch to the next layer or cell in the armor, where it meets a phase border it is reluctant to hop (much like the hand phaser vaporization effect prefers not to hop from an intruding Klingon to a starship floor). And the technobabble from "Past Tense" suggests that the ablative armor isn't exactly physical matter, but something more complicated than that.
Yuta might have been made of stern stuff, just like the rock that takes Level 16 to drill through in a practicable time. But Yuta is a small target, and anything that has any drilling effect at all should be done with her quickly enough: the extra oomph for speed that Worf selects in "Chain of Command" would not be needed. Yuta certainly is no starship wall!
Timo Saloniemi
