So why do the fat radiators cool better than thin ones then?
More surface area on the fins, the same way that tall heatsinks cool better than short heatsinks.
Note that this isn't linear scaling. A 60mm thick radiator won't provide twice as much cooling as a 30mm thick radiator.
I think EKWB has an analysis (with graphs) somewhere on their website that illustrates relative cooling performance between radiators of various thicknesses.
Depending on OP's cooling requirements it might not make that much difference which 360mm radiator one buys (assuming it comes from a decent manufacturer). Looking at the benchmark graph posted earlier, the various 30mm thick radiators are probably within 3% variance of cooling performance, not enough to make a noticeable difference.
I would focus more on radiator fan performance with a particular interest in quiet operation.
Sadly not but I was planning on running a 240mm for my cpu and a 360mm for my gpu, I'think I will go with Alphacool since I am the most familiar with their products.
I am currently using a PrimoChill 240mm EximoSM (40mm thick) radiator to cool my Ryzen 5900X (105W TDP); I previously used an XSPC TX360 ultrathin radiator but I made some case configuration changes that required me to ditch the top-mounted radiator. The 240mm radiator provides more than enough cooling capacity for this CPU.
I'm using an Alphacool NexXxos ST30 radiator for my RTX 3080 Ti (with Alphacool Eisblock Aurora Acryl GPX-N waterblock) and it provides more than enough cooling capacity for this graphics card. The GPU radiator fans max out at 930 rpm based on my fan curves so it's still very quiet.