Believe it or not, some women don't produce enough "natural" juice for sex to be pleasant, no matter how aroused they might be.
This is just like some people grow different amounts of hair, have a smaller or larger clitoris or vagina, et cetera. Hell, some people have vaginal/uterus placement that makes sex outright painful, along with many other problems or natural variances.
Just because the body is "natural" doesn't mean it does its job perfectly or to the extent the inhabitant would like for certain activities.
If the body were perfect, everyone would orgasm every time, without a condom PIV sex would produce babies 100% of the time, and no one would be able to find a partner "too big" or "too small" because everyone would meet perfect and ideal parameters.