Well, these are rebels in Myanmar. They use what they can get. The array of weapons they use is wild. I imagine the poor sods stuck with muskets are probably left guarding less-important areas, where something is better than nothing if shit goes down.
Unironically they could probably trade in those muskets 1-1 for better guns here in the US, I'm sure collectors would love to pay a fair price even for the scuffed ones.