mostly tradition, it's a cutting tool, not a thrusting one, tho i've seen some martial arts with them using a thrust it's frequently with the pommel in the palm of the weak hand doing the pushing with the strong hand guiding it if any penetration is required (soft areas like throats are an exception but harder to hit, the bend tends to make the point hit lower than intended, there are variants with guards, tulwar style hilts, etc. but the main ones are guardless as it is not designed to either parry or thrust where your hand might slip down the blade. (you'd not do it more than once). it's not a duelling blade like a bowie,usually it's whoever gets in the first chop that walks away leaving the loser in more than one piece... it's kinda like the scandinavian knives where they say only the children need a guard, adults learn to use them without.
a lot of south east asian and indonesian weapons are guardless for similar reasons, and those designed for thrusting have grips designed for that, like pistol grips, and may even have a flaired base to the blade as in kris in order to deflect an opponents blade...there are of course exceptions and guards are found on a lot of pedangs and klewangs, etc. some in emulation of western and indian swords...