It simply ignores the fact it isn't the only spell in the book. And the fact that not everybody you want to geas is a low-level hero.
At high levels you have no use for low-level beginners (except if you're a "hero enabler" like Zeus). You want to Geas medium-level heroes.
But the consequence of disobeying the Geas is inconsequential except perhaps at the very lowest levels where it can kill you using instant death; just a healing potion away. And medium-level heroes have access to Remove Curse, sometimes even trivially so (their fellow party member is a spellcaster with that spell prepared).
At the very least the spell needs to lower maximum hit points (which is an inconvenience no matter your level) and it needs to specifically say attempts to dispel the geas or break its curse fail automatically if a lower-levelled slot than the one used to cast Geas was used.
This way Geas cast as a 9th level spell lowers your maximum hp by 27 hp on average, and only a spell cast using a level 9 slot has a chance of breaking the Geas.
This is important since the expectation is that you're normally geased by a spellcaster of higher level than yourself.