Mac people do not buy RAM now because they have no option to, Apple killed this market for their own machines, I would not say it was diminishing in the Mac community and was a cheap and easy way to add life to an older machine.
You can pay Apple when you order your machine to upgrade your RAM, of course they charge. I will say RAM is probably not as important for most people these days as most machines are faster then what most people need for regularly daily life. The pro community is different though and people do upgrade and change, again because of Apples move away from this it does not happen in the Apple community anymore. Apple has stopped selling Pro desktop computers like the towers that were very popular with the G3's, G4's and G5's. RAM upgrades were easy, hard drive swapping just as easy, now basically not an option. Buy what you want at the beginning and when you need more get a whole new computer.
Storage is another thing, why would anyone want to pay forever to maintain their data in the cloud with hard drives being incredibly cheap? I use cloud storage for secondary backup, it is cheap for unlimited space but it is a offsite secondary backup of valuable and important content for me and not something I am needing to access regularly or work off of. Working off a cloud drive is not great and not something you do for pro use, files are too large and you would need some pretty amazing speeds to run anything out of the cloud. People fill up their computers with photos, movies and other things that they are now forced into the cloud or to pay a premium for hard drive space that costs next to nothing if it does not have Apple's name on it.
These things are all forced by Apple and their high prices for basic computer components. Yes they generally have better specs but not all pro users, maybe even most are not doing high end video work, or video work at all and not recording symphonies or mixing music. Specs are impressive, and are over kill for most people, pro or not.