Why oral sex has much much less risk of HIV virus transmitting related to intercourse??

For a few reasons:

-as was mentioned, saliva cannot transmit HIV and in fact may have a slightly inhibitory effect on HIV

-not only does HIV-infected body fluid (blood, semen, vaginal fluids, breast milk) have to leave one person's body, but it has to get inside another persons blood stream. That means in the case of oral sex the infected blood, semen, vaginal fluids, and breast milk need to find a way inside (simply having contact with the skin is not enough to transmit). This usually only happens with fairly severe oral skin disruptions (ulcers, burns, recent major dental work, current STIs in the mouth)

-the mouth is a pretty hostile environment for HIV as it is. The skin of our mouths is tough and quick to regenerate/heal so oral transmission of HIV occurs at less of a rate than with penetrative intercourse --- there is a much larger surface area, more sensitive skin, and a less hostile environment (in general) in the vagina and rectum.

-there are other factors but these are the basics.

That being said, I know more than a few people who are HIV+ and their only risk factor was unprotected oral sex. So, it can and does transmit through oral sex, just not in the high numbers for penetrative sex. So, it is considered a low risk activity for HIV transmission

I work in a hospital and oral sex can make one high risk for oral cancer and HPV.
HIV also is passed with saliva, and people with HIV having unprotected sex can get a different type from each other. Report It

Saliva is less likely to pass on the virus.