Because entropy. There are always and as a consequence of combinatorial necessity more disordered states available than ordered ones and as a consequence debate eventually travels through them into dissonance and disagreement but this, curiously, is also the source for more philosophy. It is a common (recursive) idiom and is endemic of and to logic, mathematics, physics and many other systems. This is how information and energy-processing systems (of which philosophy is, believe it or not, an instance) regenerate themselves and provide sustainable continuity through the invocation of an environment and transmission medium in which they are necessary, self-propagating, components. Why, however, philosophy tends to follow the arcs and contours it actually does – this is another question altogether.
Philosophers argue for the same reason that people fight wars – not for truth or glory or to win any final battle but quite simply to keep the argument going and, through it, to acquire meaning and purpose.