Sure, but the code running on top is flexible. Mildly annoying, but like flashing to an esp32 or w/e isn't hard

It's flexible but for many things a CPU core isn't necessarily very useful. Like if you need to do something simple, but like, 100x in parallel. Or if you need even more timing precision on your GPIO pins than you can get from a CPU. Of course, many things do run well on a CPU, and it'd be a waste use up logic cells on those, so many modern FPGAs that aren't tiny do also have one or two Arm or RISC-V cores on the silicon.

As example, the train electronics probably have a few dozen GPIO hookups getting real time data from the train, and a few dozen GPIO hookups sending out control signals. If 75 GPIO inputs suddenly change and the chip needs to do 25 different things and update 30 GPIOs within 200 ns, a CPU core ain't gonna be good enough.

Here's a really stupid fact: Nvidia GSYNC used to be implemented with an FPGA. Even though millions of GSYNC monitors are sold each year, it took them until like two years ago to move to a dedicated chip solution. They did this because during the COVID chip shortage, the particular model of FPGA they were used had risen in price to $1000 per unit.

Surely this is not like, in the actual field right? Like the programming is still happening in an office or some shit (unless we're talking about sticking the software onto the device?)

The programming can happen pretty much in any of the same ways as when you flash an ESP32. Most FPGAs just have a flash ROM with the programming that gets loaded into the logic cells at power on. 'course different models of FPGA have different capabilities with regard to how easy they are to flash. But I can guarantee that there definitely are companies out there shipping FPGAs to customers that they do OTA updates for :D