There was a debugger BTW, the very limited programmer's key (which most users only used to kill the current app if it crashed). It could be replaced with the more powerful Macsbug.
DOS's DEBUG isn't just a debugger; it also functions as a disk editor, hex editor, memory inspector, and general "system inspecting/tinkering" tool.
I used to be in the former camp and have moves to the latter. Consider cars. There are lots of "car people" who love to tinker with them, but most people just want to get where they are going.
It would be unreasonable to expect everyone to be a car expert, and it would also mean everyone would have to spend the time required to learn it. That would cut into time spent doing other things, which would be narrowing and wasteful. Instead of composing music or starting businesses or writing novels, people would be futzing with cars.
If you are a computer person there is more and more diversity of cheap hackable stuff out there today than ever before. The enthusiast market and the general market are not the same and are likely to keep diverging. Same has happened with cars.
The Apple ][ was also open in the sense that it shipped with full schematics and an annotated listing of the ROM (https://archive.org/details/applerefjan78)
Inside Macintosh had a high-level description of the hardware, explained the memory map and how to call OS calls, and had good descriptions of the various data structures, but didn't go as far as including full schematics or a full listing. It also was a separate thing to buy, so most users wouldn't have it.
Edit: This comment originally went on to briefly describe the Apple IIGS, then tell a long-winded personal story. But I've moved that to its own blog post:
But there were other tools, too. E.g., ResEdit for manipulating program resources (I customized many programs using this, back in the day), eventually there was also an enhanced version including an assembler/disassembler. Also, there was the Mac Programmer Workshop (MPW) including all the developer tools, and it came for free. And inside this package was finally the MPW shell, a true shell for the Mac by Apple. (But this was now quite the other way round an indirect access to the machine.) It should be noted that the MPW wasn't available for early Macs, since there wasn't room to do actual programming. Commonly, the LISA (or "Mac XL") was used for this.
[Edit] Moreover, it was amazingly simple to change the configuration of the Mac, especially with System 7 and higher. Just drag things in out of the System Folder and you had already set up the system to your exact needs.
Apple's way is inherently more idiot-proof, but makes a sharper division between developers and users. PC magazines in the late 80s and early 90s had articles consisting of short assembly-language programs the user could create with DEBUG, and I think in general it encouraged somewhat more open culture of tinkering and learning with their machines than Apple's philosophy of opaqueness.
It wasn't anything like the walled gardens of today, but I remember the effort required to even get started writing applications (or just modifying existing ones) on the Mac was significantly higher than the PC.