Advancements in machining technology and metalurgy make mass production more cost effective and of a higher quality than ever before. Additionally, CNC(Computer Numerically Controlled) Machining allows for very easy changes and option upgrades such as porting with little or no additional cost. With CNC Machinery the manufacturer doesn't necessarily need to hire more employees to increase production. (Within certain limits.) Also, machine operators are not as highly skilled as machinists and therefore do not cost as much. The real skill takes place in the initial CNC Program written to create the particular part. The machines actually have the ability to compensate for their own wear and tear when creating parts. Exact duplication has never been more accurate. The machines will even tell the operator what maintenance they need and when. They also do self diagnostics.
Another thing that is going on is that many people who otherwise would not be in the market have been drawn into the market by the fear of gun ownership rights being taken away. These people are mostly coming from middle and upper middle class, and they are willing to pay for all the bells and whistles. In mass production it's cheaper to produce all of a given model alike than to eliminate options from some of a production line. The Japanese proved it to the U.S. Auto makers in the 80's.
Also, if they are selling a higher volume of products, then the liability and other operating costs get spread over a wider base, lowering the cost per unit. Even the cost of materials drops on a per unit basis. If you look around, you'll notice the same trend in everything from home electronics to bicycles, lamps and home appliances.
Bottom line, it doesn't cost anything more to machine a beaver tail into the frame. Just a little different direction for the mill to go and very little extra material, which was probably being thrown away anyaway.
I hope this answers your question.
It's just a simple matter of supply and demand.