(being IBM, they later tried to "upgrade" to a locked-down microchannel bus, but it happily didn't go anywhere)
other things got standardized (motherboards, power supplies, peripherals, etc) and it has been going ~ 40 years now.
Price and size and overall state of tech back then didn't really allow to have several PCs in one (for some rare and pricey exceptions like PowerPC CPU extension boards, etc.). These days we can potentially have inside regular size laptop a backboard with standard bus (something like Sipeed cluster board which takes up to 7 credit card sized SoCs [1]) into which we can potentially plug various SoCs of the same or different arc. Say one SoC is RISC-V, one x86 and several SoCs with powerful NPUs - configure your laptop(cluster) for the mission at hand. Dare i say from a common bin of parts in the office (or even from public library - the SoCs are just tens of dollars nowdays, like say a game cartridge).
[1] one can imagine if the cards were inserted at angle instead of vertically https://www.amazon.com/Sipeed-Lichee-Cluster-High-Performanc...