Depending on your hardware platform, there could be valid reasons why you wouldn't want to run Frigate NVR in a VM. Frigate NVR it works best when it can leverage the GPU for video transcoding and TPU for object detection. If you pass the GPU to the VM, then the Proxmox host no longer has video output (without a secondary GPU).
Unless you have an Intel Arc iGPU, Intel Arc B50/B60, or fancy server GPU, you won't have SR-IOV on your system, and that means you have to pass the entire GPU into the VM. This is a non-starter for systems where there is no extra PCIe slot for a graphics card, such as the many power-efficient Intel N100 systems that do a good job running Frigate.
The reason why you'd put Docker into LXC is that's the best supported way to get docker engine working on Proxmox without a VM. You'd want to do it on Proxmox because it brings other benefits like a familiar interface, clustering, Proxmox Backup Server, and a great community. You'd want to run Frigate NVR within Docker because it is the best supported way to run it.
At least, this was the case in Proxmox 8. I haven't checked what advancements in Proxmox 9 may have changed this.