Because some parents believe that an older guy would be more mature, more dependable/reliable, stronger, and a better supporter.
AND ALSO…I’ve even heard some women (desi moms) say that since women age faster…it’s better for a guy to marry a girl who is considerably younger as she will retain her youth longer. Isn’t it interesting how it’s usually the desi MOMS who perpetuate beliefs that are overall harmful for women?