There are many reasons why inscription is so profitable. Obviously, the sheer amount of glyphs are one reason. The fact that materials are cheap to come by is another.
I don't farm my herbs from inscription despite having epic flying on my Paladin (who is my herbalist). I would actually be losing money if I were to farm it rather than buy it. Let me explain.
Farming herbs takes time. Lots of time. You will get a few frost lotus here and there which is nice, but wont really make up for the lost profit. On average, the high level northrend herbs give me 5-6 ink of the sea and 1-2 Snowfall Ink per stack.
I buy stacks of icethorn and adders tongue for pretty cheap. I scoop up icethorn priced under 17 gold a stack and adder's tongue when it is under 15 gold a stack. After I mill one stack I have enough ink of the sea to make 5-6 glyphs which will sell for 5-20 gold. I then sell the ink of the sea on the auction house for more than I payed for the stack of herbs. With this method, it is as if the auction house is paying ME to take the herbs.
It is not uncommon for me to end up with 50-60 Snowfall ink from 40 stacks of herbs. The first 40 ink pay for the herbs....then next 10-20 are pure profit. Not to mention what I am making off of the glyphs.
I could never farm 40 stacks of herbs in an hour. As the goblins say, time is money.