Phrase or idiom for someone that is overspecialized

A one-trick pony would be appropriate here. Merriam-Webster defines it as:

someone or something that is skilled in only one area

A person who knows everything about nothing

The following quote is attributed to Konrad Lorenz

Philosophers are people who know less and less about more and more, until they know nothing about everything. Scientists are people who know more and more about less and less, until they know everything about nothing.

Example: Ask him about the mating habits of the coelacnanth, and he will tell you everything there is to know, but he has no other topic of conversation, and is never invited anywhere.

Consider,

have a one-track mind

Fig. to think entirely or almost entirely about one subject. Adolescent boys often have one-track minds. All they're interested in is the opposite sex. Bob has a one-track mind. He can only talk about football.

McGraw-Hill Dictionary of American Idioms and Phrasal Verbs

He has a one-track mind. All he ever talks about is trains and railroads. YourDictionary

Most folks have a one-track mind, and I even want to say, a "single-minded obsession" about "note-taking".

From the standpoint of productivity, "note taking" is just a very small part of the information gathering equation.

Most folks don't know there is another important part, and that's, "note-making." Quora

have blinders on/be wearing blinders

Blinders are coverings placed over a horse's eyes which prevent the horse from seeing anything that isn't straight ahead. When wearing blinders, the horse can't see anything to the sides. This helps prevent a horse from being scared by things it can't see.

The idea is that if you're wearing blinders, you have a very narrow view of things, and you aren't aware of everything that's happening around you. If I tell you to take your blinders off, I'm really telling you to look at everything around you and make an informed decision based on all the information available, instead of just making decisions based upon what you know by only looking at a small part of the problem.

english-test.net

Not to be able to see the big picture

the big picture also the whole picture

the most important facts about a situation and the effects that it has on other things Melissa's opinions don't take the big picture into account. Usage notes: often used after look at: When you look at the big picture, a slight increase in unemployment is not significant.

Cambridge Dictionary of American Idioms

be sitting too close to the screen to [take in/see/get/grasp] the whole picture

WordReference forums

be looking through the wrong end of the telescope