Why people still buy Sterak's Gage and Maw of Malmortius together?

They're both still strong items by themselves.

Also, even though the shields don't stack, the extra passives do because they have different names. Meaning you get both Sterak's Fury AND Lifegrip whenever either shield is triggered.

Ay danno. Healthcare?

They are still good items, and when one goes off so does the other. The change just made it so they were not quite as good together as they were before, but it didn't make them completely suboptimal.

to trigger lifegrip (+20AD +10% lifesteal & spellvamp) of maw at the beginning of the fight vs damage burst instead of once you get at 30% health, and that until OOC; it just make Maw buff way more efficient. Well that's my reason to combine both.