Stereo relies on an equilateral triangle to maintain the "phantom center". You need very high quality amplification to create a soundstage that goes beyond the speakers location. You need to pressurize the room if you want a true concert like experience and that requires speakers that are big enough for your room to get it done. All of this is painstaking to dial in for the ONE MLP in the room and if you move anything start over and good luck with that. Granted, do all of the above, sit in the right spot and as long as you don’t move it sounds good as long as the material you are listening to is typically presented in FRONT of the listener like acoustic trios, piano, female vocals. All the stuff they dish out at audio shows (for a very good reason). This is not a scheme, it is just what you are forced to do because the recording engineer was forced to compress x number of instruments, voices, and room reverb into ONLY two channels. Ask any engineer if they want a bigger palate than two and see what they say (duh).
Now once you drop in a center channel no more "phantom" and you can move around the room and vocals still come from the center, what a concept. Atmos is object based, the engineer can place objects specifically in the mix. Atmos "sees" you speakers layout and will try and recreate that object at the same location. More speakers=more freedom for the preciseness the engineer employed. Less speakers (or headphones) it still works. A SOA two channel preamp (like the AudioNote M10) is $100K+. A SOA immersive preamp (like the Trinnov) is between $20K and $30 depending on how you configure it.
As for speakers a pair of of SOA speakers can easily cost more than $25K, add the amps and that can easily be another $10K. This is not a ripoff, its simply necessary to have two speakers fill a room with a high quality believable soundstage (uhhh IF you are in the sweetspot).
When you employ 5 or 7 or 9 bed channels and 2 or 4 or 6 height channels you don’t NEED all of that power. Think of the driver coverage in just two speakers with 8 inch woofers for example (16 inches total). Now think of the total driver coverage of 9 speakers (5 beds and 4 heights) with 6 inch woofers (54 inches total). You can get a completely immersive soundstage as "objects" appear in the mix where the engineer placed them in the mix or at least close if you have fewer speakers.
Now, stereo needs and equilateral triangle. Immersive audio need the layout of the specs of whatever format you want to use (dolby or auro 3d are a bit different).
Now with stereo you get what you get. With an atmos setup I can still listen to a two channel mix in stereo. But I got options to upmix whatever I want to as many speakers as I want AND every seat is a "sweet spot" as the "phantom" center is now an "actual" center, just like if someone was singing or talking from that same spot in the room, it will always sound like its coming from the center, wherever you sit.