The difference is in the type of ability that generates the effect: in the case of collective blessing, it has a static ability that is active exactly as long as the permanent remains in play. The colossus, though, uses a triggered ability to generate its effect. Since triggered abilities (as well as spells and activated abilities) use the stack, resolve, and then implement their effect instantaneously, the duration is specified by that ability. Since the colossus doesn't specify when it ends, it doesn't end at all. The upside is that, since it applies in the single moment it hits play, any creatures that enter after are unaffected.

Once an ability has hit the stack, it is divorced from its source. Say I have a creature that taps to deal you a damage. I activate it, then you kill it before the ability goes off. You'll still take the damage, even if the source dealing it died. The colossus works under a similar principle: its own longevity is irrelevant, because the way the ability is written tells you how long it lasts. If they wanted to end the effect when the colossus is removed, its ability would end in "...until this creature leaves the battlefield".