Because a fertile soil and a potting soil are two different things.
Loam is just a base soil type you'd find in a field.
Potting soil is an artificial environment that you are creating to maximize your plants potential in a small space. There's not really anything "natural" about it. Adding loam to OF could reduce it's ability to breathe and could make your soil more soppy.
There's lots of good information in the organics section about making your potting soil into super soil, it works like a charm!
Better would be adding some compost or worm castings and balancing it with some perlite.
Here is a primer on the topic by a long passed friend:
https://www.autoflower.org/threads/tlo-for-dummies-from-a-dummy.17822/
And some recipes people use:
https://www.autoflower.org/threads/tlo-super-soil-recipes.10566/