I often wonder the same. I grew up at a time, where I had to learn assembly on the C64, to get my machine to handle > 300 baud with uploads/downloads, on a BBS I wrote.
On the hardware side, it was very easy to learn how to fix said machines, with schematics provided in the back of the user manual. This level of understanding is not as easily possible today, and my understanding grew outward from that.
I think two things, the need to understand low level to work with hardware competently in the past, and that it was fun to work with hardware in the past, have led to a theoretical golden age of people who understood it all.
I have deep understanding of baremetal, right through to frontend. Part of that comes from working with computing as it changed and morphed.
People today are graduating, entering the field, but are starting at a higher level.
One other thing to bear in mind. When I entered this field, it wasn't to make money. Salaries were abysmal, and on top of that, the field was seen as "boring" and "stodgy" to a great many. And there were often even poor employment prospects!
This meant that people who worked in computing prior to 2000 or so, did so because of joy. They didn't think "Oh, I can make a good living here, I choose this job". In fact, people would tell you not to work in computing, who on Earth would hire you!
So people from this era often were in this field, because they were drawn to it not for income, or job security, but instead because it was more important to work with the fun and joy of computing.
While there is absolutely nothing wrong with entering a field to make a living, or for job security, the motivations are different.
An example here ; using a computer is different than building computing. Enjoying using a web browser, enjoying interacting online, that is not "enjoying computing". Yet most people I speak of, were building circuit boards, coding in basic, when kids.
How many CS graduates were coding apps, building webpages, for 100% fun, without any encouragement when 5?
So I think the same capability is there, it is just that most people today are not interested in computing. They just happen to make a living there, and so those enjoying computing as a primary are awash in a sea of those who couldn't really care less.