That "personal a" only applies when the person is a "direct object" of the verb (they are on the receiving end of the action), not when they are the subject of the verb (the one doing the action).
In "me interesa ese chico", the verb is interesar ("to interest"), and the subject of the verb (the person who is interesting someone) is ese chico. So no "personal a" is required there.
It might be a little confusing because in the default English translation, "I'm interested in that boy", the subject is actually "I", not "that boy". But the English phrasing has just been modified a little to be more natural; the more literal translation would be "that boy interests me", in which it is more clear that "that boy" is the subject of the sentence.
There are two different ways to talk about interests in Spanish.
The first is "me interesa ese chico", using the verb interesar. Interesar is a gustar-like verb. It's similar to the English phrasing "that guy interests me".
Then there's "estoy interesado en ese chico", which is more similar to the English phrasing "I am interested in that guy".
"me interesa en [...]" is like a mix of the two versions, which would not make sense, like "that guy interests in me".