I think this question is primarily based on a misconception about the nature of oral and genital infections with HSV-1 and HSV-2 in the modern day.
HSV-1 is sometimes described as an oral herpes virus. That does not mean it is only an oral herpes virus, just that among oral herpes cases it tends to be more common.
HSV-2 is sometimes described as a genital herpes virus. That does not mean it is only a genital herpes virus, just that among genital herpes cases it tends to be more common.
The relative prevalence is not something like "99%" of one sort or the other. For example this editorial:
Wald, A. (2006). Genital HSV-1 infections. Sexually transmitted infections, 82(3), 189-190.
points out that HSV-1 is a common cause of genital herpes:
For example, a Japanese study of women, published in 1976, documented 43% of genital herpes as caused by HSV-1.2 In 1977, a university health clinic study showed that 37% of women with clinical diagnosis of genital herpes had HSV-1 isolated.3 Among people with newly acquired genital herpes in Seattle in the mid to late 1980s, 32% had genital HSV-1 infection.4
The editorial further discusses the increasing prevalence of HSV-1 among genital herpes cases in reference to the article the editorial responded to:
Haddow, L. J., Dave, B., Mindel, A., McPhie, K. A., Chung, C., Marks, C., & Dwyer, D. E. (2006). Increase in rates of herpes simplex virus type 1 as a cause of anogenital herpes in western Sydney, Australia, between 1979 and 2003. Sexually transmitted infections, 82(3), 255-259.
and attributes this to a mixture of reduced oral HSV-1 in children, making them susceptible to genital infection as they begin sexual activity, and changes in sexual behavior to include more oral sex, especially among adolescents.
There are no studies I am aware of where an experiment is done to take individuals infected with oral HSV-1, have them engage in oral sex with someone naive to HSV-1, and find out how many infections occur. I'm sure you can imagine why. It is possible to make some inferences based on prevalence and surveys of behavior, but it's typical for people to engage in a variety of sexual behaviors and also common for partners to not know they are infected or not share this information, so it often may not be possible to determine transmission routes definitively. It's also common for these studies to focus on cases of HSV, so they may not have a comparison group of people who are HSV-negative despite partners who are positive.
Studies like this one:
Löwhagen, G. B., Tunbäck, P., Andersson, K., Bergström, T., & Johannisson, G. (2000). First episodes of genital herpes in a Swedish STD population: a study of epidemiology and transmission by the use of herpes simplex virus (HSV) typing and specific serology. Sexually transmitted infections, 76(3), 179-182.
find both that genital HSV-1 is common and that:
Of 26 HSV-1 infected patients, all but one
who answered the question about orogenital
sex reported having practised orogenital sex in
connection with onset of the infection, compared with eight (57%) of 14 in the HSV-2
group (p= 0.004).
Or this one:
Common patterns noted in patients with genital HSV-1 infections included a negative personal history of cold sores, having a sexual partner with a recent cold sore, orogenital contact, being an athlete, and cosmetic body shaving.
Orogenital contact is a major factor in the transmission of genital HSV-1. Oral sex is commonly thought to be “safer” for prevention of pregnancy and sexually transmitted infections, including HIV disease, compared to vaginal or anal intercourse. Younger adolescents, especially, think of oral sex as more acceptable than vaginal intercourse, which is more likely to occur in the future for themselves and their peers. 23 College students report having oral sex and vaginal sex at about the same rate (45.2% oral sex and 48.8% vaginal sex); however, they report using condoms 52.1% of the time for vaginal sex, but only 3.8% of the time for oral sex. 24 Women who had receptive oral sex or vaginal intercourse were more likely to be HSV-1 seropositive than sexually inactive women. 25 White race, receptive oral sex, and anal intercourse were also positive predictors for HSV-1 genital infection. 18 Orogenital contact and having a single partner over the past 2 months were associated with HSV-1 infection, whereas HSV-2 was associated with a higher number of sexual partners in the last 6 months. 13
This seems like strong suggestive evidence that oro-genital HSV-1 transmission is common, particularly for female recipients of oral sex (though the authors point out that it is not clear from these data whether that is due to differences in anatomy or differences in behavior). Preventative measures against sexually transmitted infections (e.g., condoms) are less common for oral sex than vaginal sex.