It's written into the standard, so it's hard to change. If you strengthen it, you run the risk of vendors offloading it and getting it wrong some tiny fraction

It's written into the standard, so it's hard to change. If you strengthen it, you run the risk of vendors offloading it and getting it wrong some tiny fraction of the time. The lower layer is always offloaded so it's actually the enemy: it's not an end-to-end checksum!