They physical overhead (size, weight, cost) of socketed CPU/RAM/etc just isn't worth imposing on 99.99% of users who will never make use of that capability. This is exacerbated by the increased synergy of the components: a major part of Apple's success comes from managing the interconnectedness of all things, all carefully balanced for exact purposes - not generalized for 3rd-party parts swapping (for which failures get blamed on Apple, not the cheap POS part you thought might work).

You want a lightweight high-quality device with long battery life? You're not going to get it with CPU sockets and access panels.

By now, most users have learned that computers ARE disposable: come end-of-life, vanishingly few will want to futz around with upgrading parts at cost of time spent, only to extend the life of a machine straining into obsolescence. We all dearly want to hang onto those old familiar machines, but fact is by the time a product hits that point you don't want something running the best of 5-year-old+ tech, you want the next generation with better everything.

I keep coming back to a rule of thumb: for something used heavily, daily, is it worth $X/day? An iPhone X over 3 years is $1/day. A robust MacBook used for work is certainly worth $3/day. Come practical end-of-life, it has more than paid for itself. ...and that's why Apple has made it so easy to switch up to a new machine, and abandoned notions of component upgrades. If coffee costs more during that time than the hardware in question, get a new one and sell/donate the old (making the switch even cheaper).