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Part of it is because hardware properties are always changing. The instructions available, the relative speed of CPU to memory and the various caches, how big and numerous and fast the various caches are, etc etc.

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I am curious why things can't just get better on a base that doesn't change, until the base changes because the improvements with the new base are just that much better...

Or is that why hardware properties change so much?

For a time that is what happened. Every year you would get a new CPU and that would be faster at pretty much everything than the version the year before. But then we hit the clockspeed wall and the only way of making faster CPUs was to add complexity to the internals of the CPUs. So branch prediction, micro code pipelining, larger caches, simd instructions more CPU cores ect was the result.

So nowadays a new CPU might not be better at everything then the previous version but it will most likely have more cache and some internal improvements to pipelining/concurrency.

Given this, for newer versions it can be useful to add instructions to take advantage of extra pipelining or using a different instruction that happen to be faster now.

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