A few reasons: Because a Raspberry Pi and Ebay touchscreen won't be running or supported in 20 years, while the PLC will. The IO on a Pi needs optoisolators and hardening that's built into the PLC, and it needs soldering, while the touchscreen has built-in screw terminals. And while the software sucks, anyone remotely competent can go from zero knowledge to bumble into a working, sturdy product on the PLC in perhaps 80 hours. Qt has a much steeper learning curve.
Essentially, the touchscreen is an RPi and touchscreen, plus some industrial enclosure, strengthening, and connectors all wrapped up into a product. That integration is definitely worth $400. Or, if you want the name brand stuff, the Allen Bradley version of the same will set you back another $2000.
In addition to all the features you'd have to pay someone to add, and software you'd have to pay someone to write, what are the noise immunity, temperature, shock, vibration, and emission etc. specs for a RPi? Does it have UL, CE, etc. approvals? Does it have a warranty?
Does it still have all these things after you've modified it?
A raspberry pi + touchscreen is 1/5th of that. RPi is capable of running Qt5 which would be a solid UI choice.
How the heck have they survived?