They don't - not inherently, at any rate. It's a design choice. If your aeroplane requires equal positive andf negative G capability this can be desigtned into the chosen structural and aerodynamic design solution. Very few aircraft (or more to the point very few missions usually undertaken with aircraft) need this capability, so an "asymmetric" solution is designed which is both lighter and has lower drag.
People can be made to tolerate far more positive G than negative G (G-suits only work for positive G), so for agile combat aircraft the prefered solution is one that give a large postive G capability (and also a large dG/dt) coupled to a rapid roll rate so that the aeroplane can go where needed by rapidly rotating the G-vector intro the desired direction and pulling the G. Interestingly - the same solution is used in nature by small, agile birds.