Let me try to add to the excellent answers.
Your question is legit, but it can be readily explained with scale.
Compare: "me and my brother built a doghouse yesterday - why building a skyscraper needs an architect and blueprints?" :)
More in detail:
The musicians are presumably professionals who have had much practice at this point, so why do they still need direction?
Rocket scientists can have a chief - often more senior, more experienced - scientist as well, coordinating the effort and giving general directions.
The conductor does the same, but since the score is often open to some degree of interpretation he also disambiguates and, one could say, imposes a unique vision, going so far as doing slight edits to the score.
This tends to work better than 90 musicians in a hall arguing about how fast "presto" exactly is.
So, the conductor's job is mostly done in rehearsal.
During performance, he provides visual cues which might be helpful for playing in perfect sync (although this responsibility is often shared with e.g. the concertmaster).
Other musical groups such as bands or soloists don't need any extra guidance.
This is a bit of a false analogy.
At the extreme, a 4-element crust punk band playing in a club full of drunk people doesn't require guidance in the first place, because the music is less complex, everybody plays on 11 anyway and disputes can be conveniently resolved by physical fighting :)
As for the tempi, that's what the drummer is for, keeping the beat.
During the quiet passages, you may sometimes notice the drummer is keeping the time aloud with his sticks anyway, in fact doing what the conductor can do purely visually with his baton.
Speaking of visual cues, notice how at the beginning of this video the drummer counts out the time, keeps the beat with his hi-hat and... notice what happens at 4:35! :)
Note that big bands do often have a conductor or a leader, anyway.
The function of the conductor can be better understood in an historical perspective, if you want.
The figure of the conductor (as opposed to the concertmaster, an instrumentalist which covered some of the above functions) emerged in the 19th century, when music became more complex, a greater variety in tempi and dynamics emerged and orchestras grew bigger.
Modern-day smaller chamber ensembles can still do without a conductor for the same reasons (and historically informed performers may chose to do so even with larger ensembles/works).
And on a similar note, I also tend to see orchestras with sheet music in front of them. For the same reasons, why is that?
This question can be rephrased as "why was writing invented"?
Of course, because it is more practical than committing everything to memory if you don't have to :) - especially when performing complex, non-repetitive, lengthy pieces of music with dozens of other instrumentalists where you are kind of required to follow your part exactly for the whole performance to work (whereas in a 4-piece rock band the drummer might change a fill and nobody would care much).