Apple’s soldered Unified Memory is far better than what you get with slow, antique RAM cards. It’s faster, more power efficient, and is a big part of why the M-series chips are so blazing fast. You replace the high-speed Unified Memory with RAM cards (which also would take up far more internal room on a MacBook Air and would mean you wouldn’t have as much room for things like battery), and performance would take a major hit. Better to stick with Unified Memory, and most people just buy the configuration they want. Most PC manufacturers I’ve seen using similar soldered RAM are charging close to the same, the same, or more than Apple. High-performance RAM on a high-performance chip isn’t cheap, and none of those Windows PC flunky computers using soldered RAM can really compare to the Macs on performance. And 8GB of RAM on the M-series Macs goes a very long way, as someone who owns one and does video editing, 3D modeling/sculpting, graphic design, etc, an 8GB M-series Mac absolutely compares to roughly a 16GB Windows PC, and actually beats them on many other fronts such as thermal management and not having loud fans whirring, better quality display, better sound-system, etc.