Contract ReWrite rather than Contract Extensions

[Sanchez_Mareno]

How come when renewing a contract the final year of that deal is replaced by the first year of the new deal. Why not add the new contract to the existing one
Eg a 5 year rookie deal is extended in year 5 by 4 more years. So in total should be nine years. Not 4 years and 4 years.
Hope that makes sense

It just makes it alot harder with cap room.
You can be right on the cap one year and then have to pay more for players you already have. Feels like double jeopardy