Getting a part injection molded is a pretty involved process. You have to find a manufacturer who will work with you, have a prototype mold made, do a test run, correct the mold/material/process, have a new mold made (iterate as necessary), then do a full production run. Making the molds is usually pretty expensive by itself. It’s all fine if you’re making thousands of units, but if you only need like 100 of something it’s usually not worth the time or money.

It’s a lot faster and easier to work through prototypes by 3D printing them (it is what they were originally made for), and if you need a relatively small batch it might be more efficient.