Every chain produces a fixed amount of space per unit of time. Users bid for it with fees, so blockspace behaves like any auctioned commodity: price is set by demand against near-fixed supply.
Understanding this explains fee behaviour. Fees are not a fixed cost of using a chain; they are a clearing price. A popular NFT mint, a liquidation cascade or an arbitrage race can multiply them tenfold within one block.
Example: a chain with 30m gas per block and a typical swap consuming 150,000 gas fits around 200 swaps per block. Once demand exceeds that, the marginal bidder sets the price and everyone else waits, which is exactly what base-fee mechanisms formalise.
Related: gas-fee, data-availability