Why are Jewish people not allowed to drink milk after they've eaten meat?

they do drink milk but not milk from pigs !

Jews actually do drink milk, but there are four main requirements:

1. The milk must not come from a non-kosher animal (such as a llama or a pig)
2. The milk must not be manufactured using animal products
3. The milk must not have contacted any meat or meat product, even if the meat is kosher.
4. The milk must be prepared under Rabbinical Supervision.