While there are some open-source RISC-V cores and there are some easily available computers that use open-source RISC-V cores, most RISC-V cores are not open source.

For using an ARM core, you must obtain a license from Arm and you must pay a fee for every product that incorporates the licensed core. Moreover, the Arm license may have various restrictions on what you may do with your products that incorporate the ARM core.

With RISC-V, you do not have to pay for any license for RISC-V, even if you still have to either pay the development costs for your own RISC-V core or pay for a license to use a RISC-V core designed by another company. For some companies it may be more important that there are less restrictions, so they can design a custom variant of RISC-V, with proprietary extensions.

Except for the business model, there are no advantages of RISC-V over ARM.

There are also other open-source cores besides those that implement the RISC-V ISA, for example there are several open-source POWER cores.