When you have these devices running business critical operations, businesses are usually willing to pay more for enterprise level support, which isn't really an option for Arduino (not directly from the manufacturer at least).
On top of that I have bought 2 Siemens MSD80 ECUs (400+ EUR) and maybe around 20 VDO injectors (at 250 EUR each). The reason is not that I am a huge Siemens fan, but that the quality was and still is appaling.
You can program it in simatic manager- which can convert other languages (KOP, AWL (Assembly-equivalent if i recall)) into FUP.
Which promotes using it for larger scale projects. Which will not have objects. Not have functions. Just kilometres of ladder logic, by the time the disaster management crew is called.