Except I just did those very things you call outdated a few months ago. Doubled my RAM to 32GB and added another 1TB SSD. So....you can do it. You just can't do it with Mac. Ah well. Saved about $1200 over what the upgrades would have cost me if it was a mac as well--given it was a 16 inch gaming laptop.
That's great, and I'm not complaining about the ability to do it on some computers, albeit with the minor compromises that this entails.
Generally the issues are space, reliability, speed, power-consumption and assembly costs.
Replaceable components like RAM & SSD require sockets and these are simply larger than non-socketed alternatives. In the Apple world of "light and thin" this is important. In a gaming computer that already weighs a ton and is quite thick, this added bulk of sockets are insignificant.
I'm pretty sure that soldered RAM etc. is more reliable than a socket that has mechanical contacts. I can't think of any case where my RAM or SSD has come loose, but it's a point to consider.
In the case of SoC solutions, there is clearly a performance advantage in getting everything physically closer to the CPU, and this precludes socketed DIMM for RAM. It may also be the case that socketed RAM varieties use more power (not sure if LPDDR5X is available in socketed versions?)
Lastly, it's probably cheaper to make computers that don't have precisely manufactured sockets in addition to placing them on the motherboard.
An outlier consideration is that you put the onus on consumers to know what they are doing. Do you have the right type of RAM? Is your SSD compatible with your firmware etc.? Apple likes to lock down the variables in order to maintain their brand. Imagine if someone upgrades their Mac with slow RAM and a sub-par SSD and then posts a load a poor benchmark results? It sounds petty, but in these days of social media hype and misinformation, controlling the narrative is very important.
In any case, however much I wish Apple had done something else, the fact is that they didn't. Nothing I say will change that and getting upset about it will only lead to unhappiness.
I have spent a *lot* of time as an IT professional fiddling around with computer hardware and software, and while it was educational and saved me some money, perhaps I could have spent the effort better in actually using the machines to do something more useful.
I definitely get a bit sour when I see the upgrade prices that Apple charges, but also realise that they are positioning themselves as a "premium brand" that is driven by profit. They are charging what they think their market will bear - just Capitalism 101 at work. I could just vote with my wallet and choose a Windows or Linux computer. The problem is that Apple computers are also pretty good and on-balance work better for me than other options (and I've used a lot of others over the years). So I just suck up the price and get on with life.