Because your assertion that it cost a few thousand dollars to make a yeast that makes proteins from other animals is inexact. You can make some proteins but not

Because your assertion that it cost a few thousand dollars to make a yeast that makes proteins from other animals is inexact. You can make some proteins but not all. And that for different reasons. One is that some proteins require complex machinery to be folded and modified after their synthesis (and we still don't know everything about these) and they may not be present in the new producing organism. Some proteins are toxic for the new producing organism. And then modifying an organism especially for large proteins can be really hard.