Why does gas exist, why not have opcodes cost specific amounts of wei?

https://ethereum.stackexchange.com/questions/144999/why-does-gas-exist-why-not-have-opcodes-cost-specific-amounts-of-wei

Why would a system like: Each opcode cost a specific amount of wei, and then the total transaction you pay is the sum of opcodes*opcode fee in wei + Priority fee in wei ? So that there’s no gas and all the fighting for blockspace happens in the priority fee?