Because after thousands hour of practice, anyone will understands that efficiency and time saving are more important than spending time to show off your artistic skills for nobody.
When we are in elementary school, using calculator is cheating. When we are older, calculator is a must.
If anyone want to calculate their spreadsheets by hand, then it's their life choice.
The Disney Animation use the tracing method to produce their animations.
The Clip Studio Paint has the 3d model features purely for the tracing purposes.
The concept artists use the photo manipulation method to create concept arts.
The manga artists use real photo, real location to create their backgrounds.
The 3D artists would recreate any object or character by tracing if there is any stock image.
...
Just because the tracing method can be used for plagiarism, doesn't mean that method is a bad thing.