The main reason is probably that 11-speed cassettes have narrower sprockets (so they wear faster) and if the cassette has 11 sprockets, it costs more to manufacture than a cassette with 8 sprockets -- especially considering that usually more speeds mean the biggest sprocket is larger.
I can buy 8-speed cassettes for 25 EUR, but 11-speed cassettes are 70 EUR.
A 8-speed chain costs 16 EUR, but 11-speed chain costs 30 EUR.
Thus, the cassette-to-chain price ratio is much bigger for 11-speed systems (2.33) than for 8-speed systems (1.56). It used to be the case that the ratio was even smaller for 8-speed systems, in fact smaller than 1.0, but due to parts shortage the price of 8-speed cassettes has increased recently.
If the cassette is expensive compared to chains and cassettes wear fast, it makes sense to not let the chain damage the cassette.
If the cassette is not so expensive compared to chains and the thick sprockets don't wear very fast, the most economical point to replace the chain is with more chain wear.
Besides, not everyone replaces 8-speed chains at 0.75%. I replace all chains, regardless of the sprocket count, at 0.5%.