That's a naive statement that presumes the design used today is mostly similar to the 40 year old design.
Even if it was the exact same circuit design, that's only a small part of what makes a processor and negates all of the advancements in chemistry, lithography, engineering, and everything thing else that goes into manufacturing a chip.
It's like saying "why are we still using scissors|staplers|brushed electric motors|internal combustion engines?"
What are the viable alternatives for every use case we currently use ICEs for?
Speaking as a member of a 2 electric car family, I can tell you the current EVs are not 100% replacements for ICE vehicles.
200+ mile EVs can easily supplant ICE vehicles in almost all use cases but they're still more expensive than the average selling price of a new car in the US. That says nothing about the used car market.
Even if it was the exact same circuit design, that's only a small part of what makes a processor and negates all of the advancements in chemistry, lithography, engineering, and everything thing else that goes into manufacturing a chip.
It's like saying "why are we still using scissors|staplers|brushed electric motors|internal combustion engines?"