Imagine the First Doctor calling the TARDIS “sexy”.
Doesn’t fit, does it?
The first Doctor didn’t actually KNOW The TARDIS, though. He didn’t know at all how to fly her, and up until The Edge of Destruction he had no idea that she was anything more than a machine.
You don’t have pet names for people (or sentient machines) you don’t know.
And different Doctors would have different pet names for her, probably.
That’s what I think. I’m sure each of the Doctors called her something else. The Eleventh Doctor, as we saw in The Eleventh Hour, took a delighted look at her and said “you sexy thing”.
I’m sure each of the Doctors — even the first, the one who normally called her “the Machine” — had a private pet name that he called her, when there was nobody there…