As Midnight_v said, fantasy and history have led popular entertainment to privilege physical prowess over mental acumen. Those iconic images help people relate to the material. Plus, sometimes it's just nice to have everything. What's cooler than being a powerful wizard or a deadly swordsman? Being a deadly powerful swizdmard!
On a more mechanical note, sometimes the character you are playing needn't be as powerful as humanly possible. This board notwithstanding, most games take place at a power level far below CO standard. Playing a gish (one that relies primarily on melee with spell support, rather than a primary caster with oh-yeah-I-guess-he-has-BAB-but-who-cares) is a good way to avoid having all the power of a full caster while still having more options and surprises than a standard melee character. Plus, a gish, while tough to build properly, tends to play much more simply than a full caster, so it's a good introduction to casting for a new player.