^It's a good point about the value of human adaptability, but there's little there that would require those humans to be jeopardized by sitting inside one-person guided missiles rather than in a bigger, more heavily shielded battleship. The reason for the aircraft-carrier-and-fighter paradigm is to project power beyond the horizon. There are no horizons in space. If you just want to shoot at stuff, it makes more sense to use missiles or drones. If it's a situation that requires human decision-makers on-site, then why not just send a larger ship? If it's a question of speed, a larger ship would presumably have more powerful engines and could go faster (although there would be some tradeoff from its greater mass).
Space is not empty. Star Trek
provides all sorts of horizons in space: Hiding behind moons and planets, hiding in nebulae, hiding in sensor blind spots, being invisible to sensors, and using cloaking devices are just some of the ways that fields of view are limited in space.
As far why not encase each person in invulnerable armor, even in Star Trek
there is opportunity cost. Replicators may eliminate scarcity for things like food, clothing, and shelter against weather on Earthlike planets, but they evidently cannot themselves alone solve the problem of the scarcity of exotic (super-dense?) materials required to construct advanced and magical Treknology. There are only so many Galaxy-class starships to go around. There are far more runabouts.
And even if you make the point of your spear first-class, there's always your rear that might be outflanked. Not every cubic parsec of space is equally well protected. Not every freighter has Galaxy-class shield generators (let's just say that none do).