That is very different from what old 8-bit home computers are like. You have complete control over the system, you can write self-modifying code, you can overwrite parts which are used by the OS to manage graphics modes and so on. There are no restrictions, it's just a bunch of ROM and RAM and no limitations.

All I can do on a Raspberry Pi I can do by installing a Linux distribution on my laptop. It's exactly the same thing, and infact it's far easier by using available x86 hardware.

The whole essence is to simplify things so much that you can really go down to the real metal. direct framebuffer access, direct memory address dereferences, instant crashes upon doing something stupid.

Say you buy an used C64, plug it in your TV and turn it on. What's the stuff you are presented to? 5 lines of greeting text and a BASIC prompt! You immediately get to write code the exact moment you turn the thing on! How cool is that!? No installing software, no getting keyboard or mouse, nothing. Just connect the thing to the TV and the power cord to the wall and you're good to go. Of course the machine comes with user manual explaining the various CPU features and BASIC language and so on, very simple. Nothing equivalent to this is offered these days(although the demand for such is lower too, understandably).