Short answer
Because a copy can only start after the original trade is seen on-chain, and by then the trader's own buy, plus other copiers, may already have moved the price.
Copying is a chain: the trader's transaction lands, the copy service detects it, and then your transaction is built and sent. On a fast chain that takes seconds, but memecoin prices can move a lot in seconds, especially in thin pools.
The trader's own buy pushes the price up a little. Every copier who buys after them pushes it up a bit more. By the time your order fills, you may be paying noticeably more. On the way out the same thing happens in reverse: the trader sells first, and copiers sell into a falling price.
Per-trade caps, slippage limits and skipping very low-liquidity tokens all reduce the damage, but they can't remove it.
See the checks for yourself
18+ only. Self-custodial: you keep your keys. 0.5% per trade, no minimum. Memecoins can go to zero.