The definition of a Microprocessor is the CPU. Anything else such as the GPU or Chipset is a specialized chip.
The popularity of Mobile devices has caused Chip makers to seek out SOC designs that integrate everything into the CPU.
Remember, each CPU needs it's own cooling device so this makes a multi chip design impractical for consumer level computers. Multi CPU package Desktops are available if you take a hard look around but these will have high core count CPU packages with lower clock speeds, and the Tower/Chassis will be larger, which is impractical for a regular consumers.
Currently there is a strong demand for Desktops that are "quiet" and smaller, the there is a strong demand for laptops that are thinner. Since a CPU gives off a lot of heat, the chances of SFF Desktop with a single 16-core CPU package has a higher chance of failure.
Are you talking about the amount of cores and why consumer products have offered 2-4 cores for the better part of this decade? That's a whole other question in which you will receive multiple different answers. From 2008 to late 2017, the desktop CPU package Intel offered at the $290-$350 preicepoint had 4-cores, and the affordable CPU package Intel offered at the $120 preicepoint had 2 cores. Software developers need a few years to catch up to what's being offered to the mainstream audience.