I think for some people, it's one of those things you don't know you need until you get one (or someone tells you to get one)
I'll refrain from an analysis of my own personal history in sexual exploration. It could definitely be a cultural thing. Even American culture is weird about sex and sex toys, and look at how "open minded" we are (or claim to be). There seem to be SO many negative attitudes amongst the general population towards anything besides hands and bodies being in the bedroom (and depending on which bodies are involved, even that could be looked at negatively!) Plus, the market for vibrators and etc. is so large and can be overwhelming to look at! Escorts/companions are not immune to those attitudes and feelings, even in this line of work.
And some ladies don't really think they need to add it to the toy box, having developed so much... erm.. manual dexterity when it comes to the sexual organs. Vibrators are things marketed towards women and in most encounters our pleasure comes as secondary - unless we are talking about a gent who gets pleasure from giving pleasure

Add onto that, the fact that some people feel inadequate or awkward when a toy is introduced ("isn't my tongue/cock enough to make you cum?") it's easy to forego the vibrator movement!
I could go on and on about viable reasons ladies in this industry don't always have one and why some have never used one.
But speaking for myself, I love them. I love mutual masturbation, voyeurism, and exhibitionism. I also enjoy DATY with vibrator action. Or even giving a good BBBJ while using a vibrator, in 69 (by him) or with me on knees.
Admittedly, I am a bit wary of clients bringing them- if you are going to bring one I would recommend advising the lady you are seeing so she can possibly direct you further. I have had clients bring them to appointments and clean them in my presence (and often I will clean them as well for good measure, very thoroughly.)
Wishing you good vibes. LOL!