Some buy a complete rifle because they want a factory warranty.
Some buy because they feel it is too complicated to build themselves.
Others build because they can buy exactly what they want, and spread the money spent over time. Some build so that they know how their rifle works.
these are just a few but probably the main reasons I can think of.
I build because I like to "bargin shop" the EE on ar15.com
I "saved" probably around $750 on this build by buying "used".