Because a call induces pressure to answer, and it disturbs my current plans if I answer, and setting boundaries for how long I have to speak is awkward to do, because I have to justify the other thing I'm doing is more important (because I prefer not to be interrupted), or I have to lie to be polite, to get back to what I was doing. If I was the one wanting to speak, then I'd be the one texting and arranging a time to call.