>>717408518 (OP)Because modern battle doctrine for the last 1200 or so years has always been "the biggest ship always has the most armament, thus always wins" with very few exceptions beyond surgical and/or stealth strikes, and up until WW2, was still the prevailing doctrine until the last 80 years or so. Nowadays you can have a single cruise missile take out a behemoth like an aircraft carrier fairly easily (with regards to the previous millennia) with little risk beyond retaliation or interception.
So now Sci-fi has accepted that even the largest of ships can have a catastrophic weakness to portray that, which was evident in A New Hope, and has continued onwards to this day.
This is condensing 1300 years of battles/skirmishes/etc into a few short sentences, but there ya go. Obviously there are gaps and flaws in this post, but the overarching point remains the same: this doctrine is new and is reflected in media.