I don't know that it's the reality of engineering. (Edit: in fact there are some comments for this post providing counterexamples, an interesting one is the hardware world).

It's what we software engineers like to tell ourselves because it cuts us slack and shifts the blame to others for budget and time overruns. But maybe it's also our fault and we can do better?

And the usual argument of "it's not like physical engineering, software is about always building something new" because that's only true for a minority of projects. Most projects that fail or overrun their limits are pretty vanilla, minor variations of existing stuff. Sometimes just deploying a packaged software with minor tweaks for your company (and you must know this often tends to fail or overrun deadlines, amazingly).

I know another "engineering" area where overruns are unacceptable to me and I don't cut people slack (in the sense it's me who complains): home building/renovation contractors. I know I'm infuriated whenever they pull deadlines out of their asses, and then never meet them for no clear reason. I know I'm upset when they stumble over the slightest setbacks, and they always fail to plan for them (e.g. "we didn't expect this pipe to run through here", even though they've done countless renovations... everything is always a surprise to them). I know I'm infuriated when they adopt the attitude of "it'll be done when it's done" (though usually they simply lie about upfront deadlines/budgets).

Maybe that's how others see us from outside software engineering. We always blame others, we never give realistic deadlines, we always act surprised with setbacks.