The problem with base memory has been the standards have changed from 8 years ago. Let's use the following example:
Let's say in 2016 the standard speed was DDR3-1066 for a base of 8GB, which means 1066MHz memory speeds for those DRAM chips. Apple eventually upgraded to a faster memory bus with DDR4 at 1600MHz which allows better memory throughput. That upgrade is expensive as newer standards are always expensive upgrades. Hence, the price stays the same as the more expensive standard eats into any DRAM capacity increase. Rinse, wash, repeat.
Right now, Apple is using the latest LPDR5X, not a common standard as it's fairly new and not many chip makers are mass producing it. Hence, whatever price reduction there could have been due to increasing chip densities, they went away with the more expensive standard. Ergo, 8GB stays.