Because the chimney often isn't up to par with the requirements of a modern stove. The fix is usually obvious, not draft measurement is needed. The stove manufacturers will state what they require in the manual, experience will tell you if it needs to be even taller due to elbows in the system or other structural issues around the home.So with it being so rare, why is chimney always the #2 go-to solution anytime someone comes along with a wood-burning issue ?
With this particular situation, a draft test is needed though.