"Someone as ____ as (this specific person)" is a common idiom in English. Here's how it works.
A non-specific word…
The word "someone" means an unknown or unspecified person. It's literally just a short version of "some one", that is, "some person". For example, in this sentence:
I need to find someone who can fix my roof.
the speaker doesn't have any specific person in mind. The speaker is just looking for a person who can fill a certain role: the role of "person who can fix my roof".
You can also partly specify an unknown person by comparing them to a specific, known person with "as":
I need to find a doctor as knowledgeable as my previous doctor.
The speaker is hoping to find a doctor—who is currently unknown. That doctor must meet a certain qualification: being as knowledgeable as the speaker's previous doctor, who is known.
…used to emphasize a quality of a specific person
Your example sentence uses the non-specific word "someone" and the "as ____ as" comparison to emphasize the listener's brilliance. The sentence as a whole means the same as:
Because you are so brilliant, it is strange that it is tough for you.
I would have expected that for anyone as brilliant as you, it would not be tough.
The wording with "someone" defines an abstract category of person, like the description of the doctor who hasn't been found yet. This category is defined by the listener's level of brilliance: "everyone as brilliant as you". The effect of this wording is to emphasize the listener's brilliance by blurring all of the listener's other qualities. It creates a perspective of considering the listener from a distance, as just one of many people, mostly unknown, who are equally brilliant.
There are two main reasons why people choose the "someone" wording:
It's an easy way to refer to the listener's level of brilliance when explaining something. Most other ways of expressing the same thought in English are rather clumsy.
The speaker might want to suggest that some other quality of the listener is interfering with the listener's brilliance. Other, equally brilliant people would probably find it easy, so why not you? In this case, the "someone" who is you is meant to call attention to you, the whole person, not just your brilliance. (This is not as common as the first reason.)
Here's another example of the idiom, from Dramatic Spaces by Jennifer Low (slightly edited):
With practice, all of the actors became adept at running up and down the ramp, but I imagine it was especially scary for someone as tall as John Lithgow.
The idiom means "it was especially scary for John Lithgow, because he is so tall." The idiom uses the non-specific word "someone" to emphasize that John Lithgow is very tall, and that his unusual height makes it scary for him to run up and down the ramp.