Just to name a few:
-Most studios don’t have all departments (backgrounds/CG/photography/etc) to begin with, and when they have some those tend to be smaller and not capable of carrying a whole series.
-I don’t think any other studio besides the actual monsters has a dozen episode directors and animation directors, let alone ones as trustworthy as the people they’ve got.
-Inhouse culture starts with the studio’s own animation school, it’s very fundamental. And other companies don’t have that.
-Production in Tokyo (where most companies are set) is inherently more expensive. KyoAni’s finishing the construction of yet another building right now, it’s easier for them to have facilities that can house the entire production team.
-Their model in general is way riskier. KyoAni produces very little material relative to their size, with not that many own productions and basically no work to assist other companies. Their sources of income are a lot more limited than a regular studio, so every project means a lot more for them than usual.
KyoAni didn’t get there immediately, it took many years for them to develop into that. And it’s a dream other studios want to chase too, so we’ll see in a few years.