We can't use off-the-shelf solutions because we have unique needs and requirements that no-one else has.
Now, there genuinely are cases where this is true, but it mostly happens only in very spesialised cases, such as when you are google-scale, do space engineering or something similar. There might also be legal or regulatory reasons that you must own all code in a product. Most projects are not like this. In fact almost always someone (typically hundreds of someones) has had the exact same problem and solved it. Yet people seem to keep reinventing the wheel for no real purpose. If you ever find yourself in a debate on why people should use existing solutions rather than roll your own, here are some talking points to consider.
How have you established that your needs are actually unique?
We need more performance than existing solutions can provide
We need the ability to change the implementation at will
Sometimes people just plain lie
- The reason they give to others
- The real reason