Present Continuous~ वह अमेरिका जा रही है। would be she is going to America, like in process of going doing preparations and so on..
Simple Present ~ वह अमेरिका जाती है। is she goes to America used when talking about generalizations like what she is used to do/ habits etc.
Let's see step by step
First, the word उसका means his/her.
So, Your sentence उसका अमेरिका में जाती है। which barely translates to His/Her goes to America doesn't make sense, hence is wrong.
(Note, I say barely because the postposition का in उसका doesn't agree here in your sentence with it's object noun( name of a country America here) See Mr.Vinay's comments here. The rule is when the noun is the object of the postposition, then the postposition takes the oblique case. So उसका becomes उसके.)
Yet, His/Her goes to America is wrong. Let's modify it a bit to make it right He/She goes to America. Now, the word for he/she is वह..so our sentence becomes वह अमेरिका में जाती है।→ She goes in America.(wrong again)..did you observe that in.. yes! में translates to in making our sentence grammatically incorrect! For to we use को— the rules are same as in English for when to use to/को and when to use in/में when followed by the configuration of the verb to go/जाना.
So, correcting a bit, our sentence becomes वह अमेरिका को जाती है।→ she goes to America. (this too is wrong). Well.. here's your main question I see, "why not use को" .
जाना is intransitive verb and so it cannot be used with object..
Yes, you're right.. but it cannot take a direct object. Intransitive verbs can take objects only in the oblique form i.e. they include a postposition. So, the object here is an object to the postposition (implicit postposition —to be more precise).
SoTherefore, now, coming back to the sentence, वह अमेरिका को जाती है। here अमेरिका the object of the postposition को. But, this is an implicit"वह अमेरिका को जाती है ।", here अमेरिका is the object of the postposition को. But, the postposition here is an implicit postposition and the rule is that only the objects of postposition take the oblique case but if the postposition is implicit then the object remains the same i.e the oblique case of the object is same as the object and the oblique case is not displayed. So, here it'll be अमेरिका instead of अमेरिका को.
Hence, the right sentence is वह अमेरिका जाती है ।