Shortest answer is that RISC-V is Open Source and ARM is proprietary but what does that mean?
First, it means “freedom” for chip makers. They can do what they want, not what they are licensed to do. There are a lot of implications to this so I will not get into all of them but it is a big deal. There is even a standard way of adding ISA extensions. It makes RISC-V an interesting choice for custom chip makers trying to position themselves as a platform ( think high-end, specialty products ).
If you are a country or an economy that is being hit with trade restrictions from the Western world ( eg. China ), then the “freedom” of RISC-V also provides you away around potentially being denied access to ARM.
For chip makers, the second big benefit is that RISC-V is “free” as in beer—no licenses. For chips in laptops or servers, the ISA license is a small part of the expense per unit. But for really high-volume, low-cost use cases, it matters.
In the ARM universe, middle of the market chip makers license their designs off of ARM ( not just the ISA ). This is why you can buy “Cortex” CPUs from multiple suppliers. Nobody else can really occupy this space other than ARM. In the RISC-V universe, you can compete with ARM not just selling chips but by licensing cores to others. So, you get players like StarV and MilkV that again want to be platforms.
So, RISC-V is positioned well across the spectrum—somewhat uniquely so. This makes it an excellent bet for building software and / or expertise.
It is only the ISA that is Open Source though. Unlike some of the other answers here imply, any given RISC-V chip is not required to be any more open than ARM. The chip design itself can be completely proprietary. The drivers can be proprietary. There is no requirement for Linux support, etc. That said, the “culture” of RISC-V is shaping up to be more open than ARM.
Perhaps because they want to be platforms, or perhaps because RISC-V is the underdog, RISC-V companies are putting more work into the software for example. RISC-V chip makers seem more likely to provide a working Linux disto for example and to be working on getting hardware support into the mainline kernel. There is a lot of support in the RISC-V world for standards for things like firmware and booting.
With the exception of RaspberryPi, the ARM world is a lot for fractured than RISC-V. You see this in the non-Pi SBC world for example. That said, if we do consider just RaspberryPi, things are more unified there than in RISC-V.
Overall, RISC-V may be maturing more quickly than ARM did but it is still less mature overall. This is most evident in performance. Nobody is going to be challenging Apple Silicon or Qualcomm X Elite with their RISC-V chips just yet. Not even the Pi is really at risk.
Eventually, we will also just get completely Open Souce designs that anybody can implement. These could be University research. They could be state funded. They could be corporately donated designs ( older generations maybe ). Once that starts to happen, all the magical things that happened for Open Source software will happen for RISC V as well.
Open Source puts a real wind at RISC-V’s back though. And no other platform makes as much sense from the very big to the very small.