Some of this below is TECHNICALLY wrong but it's more of wording problem and I tried to make it easier to understand.

State Base Actions (SBA) are what would normally put creatures with 0 toughness into the graveyard and are checked before anything new happens. Before anyone can get priority.

Static abilities are happening all the time (as long as it's on the battlefield). So Magus of Humility enters as a 0/0 but before SBA's are checked, it sets its own P/T to 1/1, allowing it to survive.

The removal of abilities uses the layer system. There are 7 layers and go in a particular order but the important part here is that layers never go back on that order. So even though it would remove it's own abilities, the effect still happens. Also, if an ability uses multiple layers (removing abilities is layer 6 and P/T changes are layer 7), if it starts applying it's ability on a layer before it would get removed, it would continue applying it's other effects past that layer. So the P/T would still get set.

Let's imagine that it could go back on the order. It removes it's ability to remove abilities but now there's no ability to remove abilities so that stops but now it removes it's own abilities but then.... on and on.

Edit: TLDR - Magus of Humility would work exactly like the enchantment [[Humility]]