I'm sure ZOS felt the drop rate was generous since they allowed the quests to be sold. As homestead settles this should drop the per-voucher price in guild stores. But for now, your RNG results sound low. Have you trained your characters in the things that increase their odds of a master writ?

For smith, cloth and wood this is trait research and purple motifs known. As a PS4 player doing this for 1 1/2 weeks, My observations: my master crafter knows all traits and knows all but a few of the most recent motifs. He gets 100+ voucher quests frequently...probably four in ten days. My secondaries know 7-8 traits, and they have gotten a few 16 voucher quests and a 40. My lesser crafters knew this was coming...they know like 5 styles and 6 traits. Their drop rate is pretty tiny, and they get 2-6 voucher quests when they do drop.

For alchemy and enchanting it's traits known. All of my characters know these because I used that xp to level the lines. They get 2-4 writ quests all the time. One of them never got one, and when I look he'd never learned traits in either alchemy or enchanting because he'd been a mule that leveled another way. I went through the list and learned them, and now he gets them.

I don't have enough data to do the math, but I'm guessing the range formpercent chance of a master drop starts at like 1.0% for a toon that knows nothing, and extends up to like 10% for a knowledgeable toon. It could be tighter than this, but it's probably close.