- Larger devices have more space for internal components. For example, the RAM limits in M1/M2 devices are based on the physical dimensions of the package rather than on what the chips could trivially support if Apple wanted so.
- If you have to connect many devices to the computer, having internal expansion capacity can make your desk cleaner and nicer.
- Peripherals always have to make trade-offs between connection speed, cable price, and cable length. For example, Thunderbolt is heavily constrained by lightweight copper cables.
- Sometimes the availability of chips is a bigger constraint than power. Then it may be more cost-effective to use more power than more/bigger dies. Especially as wind/solar can make power ridiculously cheap for those with flexibility in time of use.
Are we talking about a Mac Pro with expansion for RAM, SSD and PCIe slots?
I dont think what is presented by Apple as RAM limits are physical or even technical. It is more by design on what ~80% of the expected configurations would be per SoC line.
Example would be the A12X Bionic SoC that came in the 2020 iPad Pro had 6GB of LPDDR4X but months later in the 2020 Developer Transition Kit it had 16GB RAM. These were selected per use case within a given product life.
2020 M1 was limited to 16GB while 2022 M2 to 24GB. Odds are Apple did not offer higher RAM configs because it would just increase the supply chain cost without meaningfully increase units sold or revenue.
It may also decrease sales of higher-end SoC like the Pro, Max and Ultra for people like me who would have bought an iMac 27" replacement with a M1 32GB RAM and 512GB SSD. That config's overkill but if you are coming from a 2012 Core i7 then it still a great leap forward. This may be why a 15" MBA M1 was not on offer as it would decrease sales of the MBP 16" M1 Pro.
Having specific customization would counter economies of scale and force Apple to redesign parts just for desktops other than the Mac mini and Mac Studio. So it may be "cheap" to replace Low Power RAM for desktop DIMMs for the desktop product lines and even allow for DIMM slots but in terms of overall cost it would be more expensive to do so because of the lower quantity ordered of said parts.
These Pro desktops will be more expensive to make and its MSRP will reflect that unless it is subsidized by other product lines that do not share that feature.
In the PC industry units shipped annually is approx ~80% laptop & ~20% desktop for the last ~2 decades. That was a reason why Steve Jobs moves from PPC to Intel in 2006.
Mac laptops & desktops use the same SoC. That's why they have identical/near identical performance when comparing same SoC SKU regardless of form factor. They could not/would not overclock the SoC destined for the desktop to create a differentiator.