This is because 2 of your contours are part of the outer boundary of the mesh. As I don't know what you are trying to create with the mesh I don't know if you can add some mesh to either end so that the upper and lower contours are not boundaries any longer but included within the mesh. Then it would be fairly easy to do as you describe. Otherwise you will need to do exactly as you describe and edit each node.
Sorry about that.
Cheers,
Gerald
"The simplification of anything is always sensational" GKC
AC 25-4013 USA, CadImage Tools
2019 MacBook Pro 16" w/ AMD Radeon Pro 5600M GPU
OS X 11.6
2.4 Ghz 8 core i9, 64 GB RAM
27" LG 5K Monitor