The term "PLC" is a lot more specific than "computer with some GPIO ports." No one that works with PLCs would consider a Raspberry Pi to be one.

These things are walled gardens. You never see the operating system. You can only change their behavior using the vendor's software. They generally run a single program (that you write using the vendor's software) on a fixed scan cycle. They read the inputs, run your program, write the outputs, then repeat.

While you're giving up the nearly infinite possibilities that an SBC gives you, the benefits more than make up for the lack of flexibility. They run (and have parts and support available) for decades. Modules are easy to diagnose and replace. An electrician who isn't a programmer can follow ladder logic and troubleshoot problems. Integrators can quickly come up to speed and understand your code.

There's a reason companies will pay tens or even hundreds of thousands of dollars for these things.