1) It's an integrated implementation. For example, RPL makes sense when you have the brilliant HP48 keyboard, and one-key access to many of the commands. It makes little sense if you're manually typing things. The number of keystrokes would go up 3-4 fold on a PC keyboard, and correspondingly, the number of stupid errors. You can't mistype a command if it's one button, or two keystrokes through the menu system.
2) Getting all of that integration, synergy, and polish requires a huge engineering and design effort and brilliant people. One-off projects can't meet it. A lot have tried.
3) The newest HP48 has a 4MHz 4-bit CPU, 128kb of user RAM, and a 131x64 pixel display. This isn't competitive in 2022.
I'd love to have a modern HP48, with an 800MHz CPU, gigabytes of RAM, and a high-res display, but to be at all worthwhile, it'd need to have the same quality keyboard, the same level of polish, the same community, and otherwise. I don't think that's gonna happen.
Unfortunately, HP calculators peaked with the 48GX. Newer models were faster but gradually lost the polish of the older models.