Open five industrial integrators' websites and you'll find the same list: Siemens PLCs, robotics, MES, SCADA, Industry 4.0, and the logos of the same brands everyone works with, because they are the same brands. That isn't dishonesty: expertise in this field is proven on the floor and doesn't fit on a web page. The result is that a buyer has very little to distinguish someone who has already solved their problem from someone who will meet it for the first time at their expense.
The second reason is that a quote is a poor comparison tool. Two offers for the same line can differ by 40% because one includes commissioning with the operator, documentation updated to what was actually installed, and six months of post-startup support, while the other stops at «it works on my bench». The lowest figure wins the tender and loses the project: the rest arrives later, as a variation order.
The third reason is the most expensive one. An automated plant isn't a purchase, it's a relationship that lasts as long as the line does — ten or fifteen years. You are choosing who you'll call in three years when the format changes, in five when the PLC goes end-of-support, and at two in the morning when the line stops and the truck leaves at six.