If you think about PCIe we moved from a 32-bit bus (32 wires bidirectional) to x16 slots with 64 wires in total (16 lanes, 2 directions each, +ve/-ve for each). Maybe you consider AGP as the true precursor of x16 PCIE, but that was also 32 I/O lanes. So there wasn't a wire saving.
The truth is if you want very high data rates over medium or long distances then you have to go with 'serial' (the three points above) and you can get anywhere between 2.5Gbps and 54Gbps per differential pair depending on the technology. If you are willing to have shorter distances then you can get up to something like 6.4Gbps per pin and it'll use less power (vastly less power if the data rate goes down to <1Gbps per pin), but you won't be able to reach as far. Technologies like HBM use single-ended (non-differential) signaling and make up for this with huge amounts of very short wires (up to a thousand or so).
Given the constraints of the technology if I was designing RAM right now with no legacy baggage I'd chose single-ended signaling, and if I was designing a high-speed I/O slot I would choose differential signaling. So it's all quite sensible and rational.
As I understand it, the switch to serial and point to point are because of three things.
a) pin/wire count reduction --- you can't have a small chip that uses a pc parallel port, cause it needs a lot of leads, etc.
b) bus termination gets weird with potentially variable reflection and distortion at high speed
c) clock skew is hard to manage on wide busses at high speed. You gotta make sure all the leads are the same length and/or add compensation logic, and it's a lot harder than a serial connection which might have a clock right next to the data line, or might be self-clocking, so the data line includes the clock signal and it can't be out of sync.
About the only parallel bus left in modern computers is RAM. Rambus was a disaster, and probably soured the industry on a serial ram interface for some time. Note though that a RAM channel now usually has a max of two slots, but boards with only one slot per channel are common too; 4 slots per channel was common many years ago, when speeds were slower.