Because the anti-biblical "Bible alone" theory is self-refuting, practicing it gives rise to countless heresies.
Without the Church over the course of 2k years issuing dogmas that act as doctrinal boundaries, one ends up not knowing with confidence even the fundamental truths of Christianity.
We wouldn't know what the NT Scriptures were without the Church, or whether they were even God-breathed.
So that alone (i.e., sola scriptura) is what makes Protestantism a tragic and dangerous heresy.
Well I actually agree with you in part. Your definition of the problem is spot on. Christians who reject all Christian tradition are reinventing the wheel. Its like reinventing math each time you build an airplane. Your understanding never grows, you stay a caveman. The Charismatic Evangelicals reject all tradition and are in a constant state of reinvention as a result.
However there is another extreme, great institutions can build up their own independent inertia and stray from the truth over time. This was what Martin Luther's "reformation" was all about. He found that some of the Church's long standing traditions were in direct conflict with the scriptures. He wanted a great ecumentical council where he could debate to make his case and hopefully reform the Church from the inside. He got the Diet of Worms instead, the rest is history, the Great Schism that followed is the fault of the Roman Catholics.
So yes Sola Scriptura, Scripture is the ultimate authority. Longstanding institutional knowledge is necessary for interpreting the Bible, but it can stray from the truth of the Bible, which is what Luther was trying to fix.
