They are essentially a copy-paste version of the ETH source code. Their fees are lower because they increased block height and reduced block time but that causes a whole lot of problems when you need to handle thousands of transactions every minute.

BSC has broken down at least 10 times this year. Solana got restarted (so much for decentralization) and even Avalanche sucked for a few months due to insanely high fees.

Copying Ethereum is easy, improving on the concept is not.