Should I draw over all imported lines?

I imported this CAD design and made the changes I wanted to make it 3D. Now I want to add a guard rail to the sides of the bridge with profile builder. I know I have to select the line to add it, but my line is not continuous so I cannot select all of them at one time.

Since I do not know the reason behind this, I thought I ask to understand why this happens? The line I imported from CAD was just one continuous line, so why now I cannot select all of it together to add my profile? Should I draw over it again??
Via derilio.skp (1.1 MB)