People are perfectly capable of understanding very large programs even from reverse engineering.

The issue is that things change so quickly that generating such knowledge is economically useless.

I have this discussion with my CTO all the time:

"I'd like you to go do <X>".

"No, please give that to <junior person>."

"But he's junior. I want someone senior to do it."

"The half-life on the knowledge to do <X> is weeks to months. Senior people will be no better at <X> than a junior person, and our junior people are quite capable precisely because we trust them with things that other places would consider a "senior" task. The first couple of weeks are going to be spent on Google, Slack, IRC, mailing lists, forums, and anything else figuring out all the ways that thing fails and how to debug it. After a week or two, they won't be junior at it, and we'll make them give a talk about it if we deploy it."

Sometimes I still have to step in, and that's fine. But I avoid learning software things with short half-lives as much as possible, nowadays.

Of course, that makes me an out-of-touch, crusty, old fart to my juniors.

But, they are smart enough to acknowledge that my code always seems to be so much more reliable than theirs. And that's fine, too.