You're assuming most people have a clue what they are doing and are best informed. They are not.
The second reason is that it is not yet FDA approved and not as readily available as the pills.
Lastly, a number of topical finasteride studies only came out in the last 3-4 years. All very positive with no reason to doubt them.
Topical finasteride mixed combined with minoxidil is the way forward for anyone wishing to avoid the potential debilitating side effects of the oral form.
Study from June 2019:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6609098/
RESULTS:
Seven articles were included in this systematic review. In all studies, there was significant decrease in the rate of hair loss, increase in total and terminal hair counts, and positive hair growth assessment with topical FNS. Both scalp and plasma DHT significantly decreased with application of topical FNS, however no changes in serum testosterone were noted.
Study from 2018:
https://www.ncbi.nlm.nih.gov/pubmed/29601622
RESULTS:
Seven articles were included in this systematic review. In all studies, there was significant decrease in the rate of hair loss, increase in total and terminal hair counts, and positive hair growth assessment with topical FNS. Both scalp and plasma DHT significantly decreased with application of topical FNS; no changes in serum testosterone were noted.
Study from 2016:
https://www.ncbi.nlm.nih.gov/pubmed/26636418
CONCLUSIONS:
The novel finasteride 0.25% solution applied o.d. at the doses of 100 and 200 μL results in an appropriate inhibition of scalp DHT potentially minimizing the untoward sexual side-effects linked to a systemic DHT reduction.
Study from 2015:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4314881/
Results:
Of the 45 patients who underwent a continuous treatment for Androgenetic Alopecia, 84.44% maintained a good hair density with topical minoxidil-finasteride combinatio. Of the five patients who discontinued oral finasteride for 8-12 months, four demonstrated good improvement in hair density when treatment was resumed with topical minoxidil-finasteride combination.
You get the picture.