Because SO MANY LINES over SO MUCH GAME.
It costs them tens of thousands of dollars to do the voicework in a game like Skyrim, which had a lot of repetition as well as long stretches of no real voice acting.
Now, in a JRPG, you have line after line after line after line after line after line after line after line after line after line after line after line after line after line after line after line after line after line after line after line after line after line after line after line after line after line after line after line after line of text stretched over as much as a hundred hours. Tens of thousands of dollars? Try MILLIONS, especially if you voice minor characters. Plus, you only have so many hours in a sound booth each day (and each one you rent costs MOAR MONEY), so it could very well delay the release.
That's why so many unspoken words. If you like dialog but don't like reading it, try old adventure games.
Also, every time someone complains about having to read, be it in games, foreign movies or whatever, I'm always reminded of Pamela Anderson having lunch with Paris Hilton, and Paris complaining that she hates reading when given a menu.
A menu.
As Pamela said afterwards, "I'm blonde too... but come on."