Firstly ARM processors are simpler to produce, with simpler circuitry (RISC!), although the number of transistors tend to be similar between the two platforms.

Research costs are also very expensive for x86, with 2 vendors (Intel and AMD) competing savagely in many different markets such as high-end gaming, servers, ultrabooks and low cost PCs. However, ARM (the company) focuses on licensing the designs and instruction sets. So their research costs are split among many licensees while ARM focused on what it does best: designing chips.

That in turn translates into many vendors who can focus on improving manufacturing efficiency, procurement, logistics and sales. x86 vendors have a wild range of manufacturing lines to keep up.

Finally, the laws of supply and demand play a huge part. The ARM market is 2 orders of magnitude (!!) greater than x86 as far as raw number of processors shipped, ~370M vs ~32B chips in 2022.