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<blockquote>I want to know why the top-of-the-line CPUs and GPUs from Intel, NVIDIA, and AMD are all bragging about the fabrication process (7nm and 5nm) and trying to be consistently smaller.</blockquote>

It's marketing hype intended to make them look more high tech than the other guy.  There is little reason the end user should care about the minimum feature size of the process.

There are reasons for making the features smaller.  Smaller features means more transistors fit in the same area.  That means the same chip is smaller, uses less of the wafer, and has a smaller chance of hitting a defect in this wafer.  Smaller features can also allow lower power dissipation.

There are also drawbacks to smaller transistors.  The difference between the on-state and off-state gets smaller.  That together with decreased voltage bumps into limits of high on-state resistance slowing edges, and low off-state resistance causing leakage.  The leakage of a single transistor is tiny, but with literally millions of them on a chip, it adds up.  That leakage times the power supply voltage is power that is always dissipated.  Now consider that cramming more transistors that leak more into the same area increases the power density.  Pretty soon you approach power densities of common soldering iron tips.  No, seriously.

Again, though, none of this matters to the end consumer.  They should only care about specs like speed, power requirements, cooling requirements, and price.  What technobabble was used under the hood to achieve them is mostly irrelevant.