Firstly, because it's incorrect English (when the subject is "she", the verb form is "sees" with an -s at the end), and secondly, because it's not a correct translation of the German.
sie sehen means "they see" and Sie sehen means "you see" -- at the beginning of a sentence, where the first word is capitalised, you cannot see the difference.
But it cannot mean "she sees".
"she sees" is sie sieht.
The sie form that means "she" almost always takes a verb form ending in -t; the sie form that means "they" almost always takes a verb form ending in -en.
Here, you have one in -en and not in -t.
Mizinamo, why wouldn't She Work instead of They?
Because the verb form is wrong for that.
"she" verb forms end in -t, "they" verb forms in -en.
So "she sees" is sie sieht with -t, "they see" is sie sehen with -en.
Since this sentence has sie sehen, it can only mean "they see" and not "she sees".