For example: you can't just throw a Birdnet-Pi (https://birdnetpi.com/) image on a normal PC and run it. There is plain Birdnet but it doesn't have any of the automation or the web interface for use Birdnet-Pi does. Instead it's pages worth of complex multi-argument commands you'd have to customize and you'd have to port the web interface yourself.

Since most people want rpis for little projects like this a normal PC massively increases the complexity.

High prices are because they've been getting scalped on the secondary market, so I wouldn't say that means they are "available."