Because voice acting is more expensive than just the payment for the actors.
It's not just the voice actors you need to hire. First, you need to find voice actors which are suitable for your roles. That means you will have to do a casting with many actors, which takes you a lot of time. Then when you have picked the actors and made contracts with them, you need a professional sound studio with professional sound technicians to record their lines. These can be quite expensive too.
Also, voice-acted lines are a lot less flexible. Say you find out during pre-release QA testing that a certain line of text is confusing, doesn't have the effect you thought it had or is just plain wrong after you changed a few things in the game. When it's just text, then changing it is just a few keystrokes. But when the line is voiced, you need to re-hire the actor (hope he got time in his schedule), get him back into the studio and have him re-record that one line.
There is the problem with releasing additional content after release. Artists, writers and programmers are replaceable, but voice actors are not. When the voice actor for an important character got a different obligation, your whole DLC project might have to be cancelled.
And then there is the problem with dynamic text. When you have procedurally generated sentences like "Pick up [item] from [person] in [location]", adding a voice-over means you have to record the segments individually and then cut them together at runtime, which can sound quite strange and artificial. When your game is fully voice-acted but that line is not, then it might sound quite strange, so you better cut down the voice acting in general.