What's the reason behind that? Probably just keeping existing, working designs and thus saving R&D costs? Sounds like beating a 10 cent risc-v core would be hard.
Edit: When I first read your comment I had to chuckle a bit, because I got the image of a multicore SoC with 6502 6502 cores. That would be a fun project for my FPGA hmmmmmmmm ...
If your chip needs very basic processing, and you're going to sell it for less than a cent per chip, you need your core to be basically free.
Sure RISC-V is very inexpensive, but for parts like these risc is massive overkill and as a modern core, you'll be spending a lot of die area on a core that should really be an ASIC, but designing an ASIC is expensive so you can just slap a 6502 in that bad boy and call it a day.
On the other end of the scale, you'd be surprised to learn that many chips you don't ever interface with as a user contain even ARM cores. These are usually ASICs in high-end products.
You seem to underestimate how small a RISC-V core can be.
Take a look at SERV.
6502 has 3510 transistors. SERV at CMOS occupies 2.1kGE.
SERV is smaller than 6502.
And that count doesn't include SERV's register file.
In that case, I'd rather have RISC-V's programming model.
SERV is smaller.