In my experience, well in excess of 20% of devs would make such claims and/or behave like this if given free rein, perhaps even a majority. This is especially true if you take "dev" to mean "anyone who commits code". The exact percentage will of course depend on the nature of the organisation and the incentives set by it.

I will give you that eventually they will take a bunch of time on something and have to explain furtively "it's more complicated than you think" but they may not even realise at that point why it's getting harder.