Software is re-written most often for political reasons, namely for new developers to leave a mark, and put a good line in their resume. Everyone likes to read &quot;designed and implemented&quot; on a resume than &quot;maintained and fixed annoying bugs here and there&quot;. We know what looks better, managers know who they&#x27;d move to the next round, and so on.<p>The only thing is, if there is no software to be written software developers will always find a reason to write software. Because they get paid full time, usually above what other professions get paid, to well, ... write software.<p>I think other reasons such as &quot;it is slow, outdated, not written in &lt;latest-language-fad&gt;, bad UI&quot;, are often brought up and used as excuses. But underlying reasons are a lot more personal and subjective.