I don't believe you are correct in stating that animal products are cheaper, or that commercial agriculture prefers it:

ers.usda.gov/amber-waves/2023/…

Issues with the product:

Despite these attributes, there are economic and environmental challenges to expanding the use of manure as a fertilizer. With a low nutrient value-to-mass ratio, manure is more costly to transport, store, and apply than chemical fertilizers. In addition, livestock production tends to be geographically concentrated in the United States, and, in certain regions, animal production generates more manure-supplied nutrients than are needed by nearby cropland. That means farmers often must transport manure longer distances to match its nutrient value with crop needs. Specifically, the nitrogen and phosphorus levels in manure often do not match the nutrient needs of crops, so farmers still must use chemical fertilizers to supplement nutrients from manure. Also, applying enough manure to meet a crop’s needs for one nutrient has the potential to create an environmental hazard from the unused nutrients left on the soil. Excess manure nutrients can leave the fields via run-off and degrade water quality, or they can enter the air.

Manure is not widely used:

A recent study by USDA, Economic Research Service identified opportunities for increasing the use of manure as a fertilizer. In 2020, farmers applied manure to less than 8 percent of the 237.7 million acres planted to seven major U.S. field crops.

Manure use is dependant on local industry:

Manure is expensive to transport, and local animal production largely determines the type of manure applied to regional crops. For example, because most hogs are produced in the Midwest, hog manure is applied predominately to corn and soybeans. Most chickens are raised in the Southeastern United States, so most animal waste applied to crops grown primarily in the South, such as cotton and peanuts, originates from poultry farms.

I think most farms use synthetic fertilizers but every organic farm I have worked on and every organic farmer I know uses some variation of the above as their primary soil amendments along with plant compost and worm byproducts.

You didn't mention your location but for example the USDA definition of organic required that 'most' synthetic fertilizers are not used. I'm not questioning your experience, I'm just stating that by nature of being a farmer seeking certification as organic they choose to exclude most synthetics for reasons of certification - not necessarily because it's less expensive or 'better'.

usda.gov/about-usda/news/blog/…

Produce can be called organic if it’s certified to have grown on soil that had no prohibited substances applied for three years prior to harvest. Prohibited substances include most synthetic fertilizers and pesticides.

Please be aware that this is a vegan gardening community. You've provided an answer to to OP based on your experience, which is fine but please understand that promotion of the use of animal bodies in gardening is not welcome.