Nowadays i think, unity-type engines are not the cause, its more like the symptom.
People who were active here 10-20 years ago, already researched, learned, and implemented everything they needed, unless they want to switch to some newer api or technology they dont know, they dont have too much reason to be active on a forum.
Gathering information from the internet and online archives is now really simple, even beginners dont really have to use forums and chatrooms to discuss technical things.
But there are less forunate reasons as well. The tiktok generation lacks a lot of abilities due to the low quality of modern education. For example, even college students dont understand, what a file system is: https://www.pcgamer.com/students-dont-know-what-files-and-folders-are-professors-say/
Putting kids in front of smart televisions to sedate them with train videos (or even so called education materials) are not going to make them computer-pros or anything, it will just cause mental degradation, and distorted morals, distorted life-view, and the result is: not knowing what a file system is. In contrast, in the 90s you had to be scientifically prepared even to use a computer, because you had to do minimal maintenance on your computer even to be able to turn it on once or twice per week - for example, pushing back an ide cable to the hdd if it fell off, or figure out how to keep your dying floppy disks alive with the help of scandisk. This was true for the software side as well, which also gave enough competence to be a basis of programmers to grown. Nowadays this is different, a year ago i have seen a young guy who told me, that programming is merely international chitchat, and he is going to be the king of programming, because he can communicate in two languages.
Newcomers be like: int, float? arrays? char mylevel[64][64]?! fopen?!?!?! xcreatesimplewindow? glvertex3f?! malloc? Are you kiddin with me? Give me that magic wasd model viewer ,,game engine'', because i will not waste five days of my life to learn the basics! Just let my fly in around some random models, and 99% of the game is finished. No one can stop me now (=
Even i, as a game engine developer, ended up doing a game engine where people doesnt have to write code or scripts, doesnt have to do models, just throw image files with a given name to given directories, and the game maker automatically generates the entire game, the maps, and the 3d models from the image and text files.