Third party lens makers have to compromise in some way to sell lenses for less than an equivalent camera maker lens.
Lower quality materials, less accurate lens figuring, less optical aberration correction, broader materials and manufacturing tolerances - all contribute to lowering the cost of producing a lens.
Also consider that pretty much every major lens maker has 3 grades of lenses - consumer grade, prosumer grade, and professional grade.
There is little point in comparing a consumer grade lens to a pro grade lens.