Why not erase those numbers?

When in a local building recently, with exposed steel beams, I noticed numbers written on the beams. I know why they were there. It has to do with the construction process, sort of like assembly instructions. What I wondered about was why some one did not remove the numbers. They are exposed to view and are rather unsightly. Just a little thing of course, but it make me wonder why the people doing the construction or those checking the installation would not have had them removed as part of their quality control. Nobody is perfect I guess.