Reliability, ruggedness, flexability, serviceability, lots of ilities.
The line of switches I linked lets you snap on independent contacts (both normally closed and normally open) that are all actuated by the same button, as well as leds in various configurations, to a baseplate that fits in a standard hole in the panel; the button itself can be swapped out from that baseplate for various reasons (recessed, illuminated, a rotary switch instead of a button, a giant emergency stop button, a 2 or 4-way joystick, etc). Various ratings on the whole mess for dust-proof/splash-proof/water-proof/high-pressure-wash-proof, which is one of the places where the cost starts to go up. Button materials range from relatively cheap plastics up to stainless steel, and potentially heavy duty enough to survive and continue functioning after blows that might shatter a cheaper button.
And you'll be able to order compatible replacement parts 20 years from now. That's a big part of it too.
Promises and history and network effects. Generally you more or less buy into a system of instrumentation. Obviously you can always run wires from terminal A to B across brands, but you can work _much_ more quickly and neatly when working with the standard-for-your-shop stuff. Your shop will have oodles of terminals and buttons and relays in stock, and know exactly where to get the oddball stuff, etc. The resellers and supply houses all keep inventories. If a part does go end-of-life, you'll probably get an email if you've ever purchased that part, advising you to make final purchases for the long term if needed, etc.
Sometimes I buy $300 buttons, they’re emergency power off buttons that are engineered not to fail. Throw in certification, warranty, and middlemen combined with low volume and you’ve got a button that sells for $300 at a supply house.
When you slam the EPO button to shut down the boiler, x-ray machine, server racks or whatever else needs to be powered down immediately in emergency situations, failure to actuate is not acceptable.
