Difference between Uniswap swap tax and erc20 transfer tax

I don’t really get why memecoins have the UniswapV2 contract included to tax on every swap instead of using the ERC20 transfer event for taxing.

Can someone explain me why they do so?

Maybe they deployed the contract before thinking on that and they can not change it now?

Can you give examples/more details? You can also contact Uniswap or check their docs.