This is one of the main reasons (the other is the number of PCIe lanes) why high end desktop and server CPUs have like double the number of pins and so much bigger sockets as compared to consumer desktop CPUs.
And as for DRAM channels, typical cheap motherboard has 2 channels and 4 slots, it should not be super difficult to add 2 more channels.
If you have a desktop CPU with 2 memory channels and 2 DIMMs per channel, then the 2 DIMMs on each single channel share all of the address and data lines, there's just a chip select difference to pick which DIMM you're addressing. There is some extra loading on the lines, since you have 2 DIMMs, hence the supported speeds are usually slightly lower, but you only add 1-2 more actual PCB traces to have 2 DIMMs per channel vs 1 DIMM per channel. In contrast, adding another memory controller would add upwards of almost 100 additional PCB traces from the CPU.
Yes they spent those costs to switch from DDR4 to over-priced DDR5 and I suggested the cost could be spent on adding more DDR4 channels instead.
In addition to cost and possibly market segmentation, presumably the additional power consumption wouldn't be worth the tradeoff for laptop parts. If you want high channel count you've always been able to purchase power hungry datacenter gear. You can also pick up surplus 10 GbE or even 100 GbE fiber NICs to link up your beowulf cluster. You'll probably have to run a few new electrical circuits and a small bit of HVAC if you're installing this in a residential home but it's entirely doable to cram 10 kW in a closet if you feel like it.
Alternatively you could just rent cloud instances by the hour to avoid the hassle.
Isn't adding pins kind of expensive?