As an example Apple designed their processors from scratch, they do not use any core designs provided by the ARM holdings corporation. Yet they still have to pay ARM a lot of money because they use their instruction set.
The instruction set is a very sticky thing. All the software gets written for a particular instruction set, and people like to keep using previously written software. Thus, software stickiness causes CPU ISA stickiness which gives companies that control that particular ISA a lot of power. And they of course monetize that power by charging extra for their CPUs.
With an open ISA, anybody can make processors that use the same instructions, which means that your software should work on any CPU designed for that particular ISA. Thus, all the talented hardware designers in the entire world can work to make RISC-V processors without asking for permission. They are free to charge for their designs or make them open. And if you write RISC-V code, it can execute on any of those processors or any future ones anyone decides to develop. (There is a bit of complication here in that the RISC-V ISA includes several different combinations for instructions for different use cases, but they are all open).