A minute of talk time consumes about 200kB per minute full duplex for typical mobile phone quality. Let's add 100% packet overhead and another 100% for better quality, so we end up with 800kB per minute. Then a gigabyte is 1250 minutes with excellent quality, or about 3000 minutes at normal quality. Do you really think anyone is going to wonder how many minutes they get out of their data plan when loading a typical web page is equivalent to 5 minutes of talk time? SMS makes this even clearer: Do you worry about the number of instant messages you can send with your data allowance? No, you know that it's "enough".
That's my point. Some people don't use their phone for web, ever. Why should they want to do this math? For many of them, they just want a straight and simple answer, even if it actually gives them less service. I can understand, for example, that a well-compressed data stream* will use a variable bit-rate compression, that if combined with efficient background noise reduction could result in conversations using a highly variable amount of data, based on how much of it is noise and how much is silence. Still, many of the people in my community would rather hear "you're getting 450 minutes of talk time" than "you're getting 2.5gb of data usage and your typical conversation uses anywhere from 64-128kps, meaning that you are getting 650-325 minutes", or the more likely advertising response of "you're getting up to 700 minutes"
Of course, I do live in a conservative community.
* And I don't know if common voip solutions do this