ELI5 ethereum high gas

Why isn't gas a fixed amount instead of a percentage? The way it's set up now, the more popular eth gets, three more expensive it is to use, leading to a value drop and the idiotic cycle begins anew.

How is that a path to widespread adoption and microtransactions? Nobody will ever use an eth debit card to buy a $1.99 Coke is the gas is $380.00.