Estrogel is usually applied to the wrists or abdomen so that it absorbs well and changes the E levels. Are you sure it makes sense to apply it to the scalp? What doses do you use, what is the result? I often hear about the advisability of applying the gel to the scalp, but have not seen the results and evidence that it is more effective and has a local effect. If it has a local effect on the follicles, even in small doses, it should quickly grow hair like from HRT, but I have not seen such results.
Same reason why betamethasone works for site application when growing back hair, which comprised a pretty huge part of why I recovered from a significant level of hairloss in 2007/2008.
You're not trying to increase endogenous levels of estrogen with estrogel, that's not the point, the point is that just like betamethasone, with topical application you're monopolizing the receptor site either with an agonist (as in the case of betamethasone) or you're directly applying estrogen, preferably in its most powerful form (estradiol) to the receptor site to offset any influence by androgens and to downregulate androgen receptors (which estradiol does).
I didn't understand 10 years ago why my hair grew back so thick, but somebody here pointed out that certain corticosteroids increase estrogen receptor sensitivity by a significant amount. That's what incites hair growth.
It also follows the same principle behind using topical finasteride, yes finasteride does have 5AR inhibition properties but it does something else when it is merely present in tissue on top of simply preventing the conversion of testosterone to DHT in the blood.
I don't know how well understood it is, but I believe this phenomena explains why some people experience post finasteride syndrome, because the drug has the tendency to accumulate in the tissues of the person who is using it. For whatever reason whether it is site-specific DHT inhibition or some other mechanism, when it accumulates in bodily tissues, it stops the effects of androgens cold, which is why some people who take finasteride for long periods have serious dysfunction of androgen related organ function (such as one's dong).
This is why topical finasteride works, not because you're absorbing it into the bloodstream, but because it is saturating scalp tissue and staying there, which is why many people who cannot tolerate oral finasteride use it this way. The effect is the same without it going completely systemic.
It is also known that finasteride builds up in bodily tissues when taken orally and that even a low dosage can remain in the body for up to a month.
TL/DR, you're directly interacting with receptors when topically applying products, the idea that you have to completely and utterly destroy or radically transform your body chemistry is not accurate for many people unless you're straight up destined to lose hair.