“It’s probably Matt Mercer’s fault.”
If you are a orc band that got attacked by these interlopers and they then ran off after you defended yourself... you don't need to go chasing after them. You let them run off, and then you pack up your gear and head off for safer pastures. Why continue a fight that you don't need to get into?
For the XP! Just think how surprised the next batch of murder hobos will be when they discover you've all leveled up and now have the Great Weapon Master feat.
1) If you're an orc band those adventurers probably look wealthy.
2) Whenever monsters flee we (as players) worry that they'll come back with even more of their friends. Why wouldn't monsters have the same thought?