If you watch the blades of a pair of shears whilst they are slowly closed
you will see that the point at which the blades only touch moves along the
cutting edges of the blades. It follows, that if the blades are dead
straight and touch all along their length, the shears will not cut.

A decent pair of shears will have a very strong spring washer at their apex
thus allowing the ever so slightly slightly bent blades to part as the
cutting point moves down towards the tips of the blades. Such shears will
go for years and never need sharpening.

G