Because if we don't understand how animals work yet (we don't) then we can't really design good replacement non-animal systems.

And if the non-animal system isn't good, then you're just wasting money and plastic

The interconnected of organs previously thought to be independent is just starting to be uncovered for example

I'm all for in vitro and cell culture models, but they don't replace animal models, unless you want to go back to running experiments on humans