I started watching Salem Techsperts on youtube recently. He sarcastically calls himself the greatest technician that's ever lived, and calls himself stupid a lot.

I think part of this sentiment is probably because working on "mechanical"/"logical" things requires you to observe events, notice what is happening, measure or react to them in a calm and sensical manner and resolve issues by repeating this process over and over again. You lay the foundations of this by having previous experiences in the domain.

So, when you mess something up, it does feel like you are an idiot, because the process feels like an obvious thing that anyone should be able to do. However, many people fail one of the previous steps I listed.