It’s easier to make your hand into a pretend vagina than a pretend penis (and use it on yourself), is at least part of it, I think.
I also think that when someone says “sex toys”, most men think they mean dildos, whereas (I would guess?) more women own vibrators than dildos. Or at least, a sex toy for a woman isn’t necessarily a dildo. Small vibrators are popular for the obvious reasons that they’re easily portable and discreet and that they can deliver a highly intense stimulation to a very localised area, which is just right for clitoral masturbation.
There’s a funny kind of reversal of the usual sexual double standard though, which is that for women, owning a sex toy is about as controversial as owning a teaspoon, whereas the idea of owning a pretend cunt made out of latex fills me with creeping horror. I guess a part of is that every man is born with a perfectly usable sex toy at the end of his right arm…and yes, a lot of women masturbate using only their fingers, but I guess the female anatomy is more conducive to using an artificial toy to help things along. Possibly also it’s because dildos have been around for thousands of years, I mean I know you can get ‘high tech’ ones these days but the basic idea is pretty primitive.