sreel breake lines will rust from the inside out because brake fluid will absorb water. When you use the brakes enough that the fluid gets hot, the water comes out of suspension. Hard for the dealer to see inside them.
Not trying to start a big thing here, but this has been fairly well proven to be a myth as far as corrosion is concerned. If it were true, all of the stuff out there with original brakes lines for decades in the past would have failed by now. Case in point, our '74 GMC was delivered new in November of '73. It still has it's original brake lines but is ready for its 3rd body. It was Ziebarted when new so the factory lines have held up nicely for 47 years and counting (obviously wasn't effective for the sheet metal). I can document that through the original owner (my Dad). My Dad never replaces any fluids other than engine oil unless something is leaking so I'd guess it's maybe on it's 5th brake fluid ever, which is terrifying for a whole variety of reasons.
A less extreme example is my '91 Mustang. OEM lines are fine after 29 years of garage life and no winters. I'm the 3rd owner, have had it for 14 years, and I see no signs of any of them ever being replaced. I haven't replaced the brake fluid in it in at least a decade as it's been in storage status for a variety of reasons and I have no fear at all of the lines spontaneously failing.
However... I bought a 1997 Oldmobile in 2000 and by 2002 every line (brake and fuel) had rotted off the car, including the fuel tank sending unit. Modern GM just uses some sort of ultra-junk steel for their lines. That's also why I haven't spent much money with the general since then...