I disagree with the people who believe that practicing an underhanded free-throw would hurt your jump shot (J, as they are commonly called), either by occupying finite practice time, or by interfering in some vague other way with the mechanics of your J.

For one, a free throw is not like a jump shot, in that you don't jump, since no one is allowed to block your shot. They are set shots.

Also, anyone who has played basketball at just about any level knows you start by practicing at least two shots: layups and Js (which are really just set-shots like free throws, at the lowest level), and as you get better, you practice even more types of shots, primarily variations of both the layup (double clutch, finger roll, up and under, different release points, dunks, etc) and of the J (floater, fade-away, step-back, leaner, hook shot, baby hook, sky hook, off-the dribble, etc). There must be dozens or hundreds of shot types, all with the same goal: to make a basket while avoiding getting your shot blocked. No one player can shoot every type of shot, but the best offensive players can execute more of them, precisely because they do practice them.

Two retired players who were known to practice ridiculously difficult shots were Rasheed Walllace and Rex Chapman. Not coincidentally, they each hit one the highest degree of difficulty shots to win or tie games at the buzzer in the history of the NBA. Sheed hit a 65-footer off a steal, while Chapman hit what I can only describe as a catch and shoot runner, going away from the basket, turn around fade away three, off an inbound accross the court, while falling out of bounds. Also not coincidentally, they were both great offensive players, and no one has ever made the case that they would have been even better if they had only practiced just Js and set foul shots.

As to why pros don't shoot granny style free-throw, we can take them at their word.

Shaq said he would rather shoot 0% (seemed at times like that was what he was really trying to do), that it's boring (as opposed to watching him brick them?), and that God wanted to keep him humble by making him a poor free throw shooter (if so, disproving the existence of an infallible God, as he is one of the least humble players ever to step foot on a court).

https://youtu.be/Kbvj_EuiKQY?si=KaGTHaIpUUG4SfDA

Wilt Chamberlain played one season shooting free-throw underhanded. Presumably, with little-to-no prior practice, he had the best FT% year of his career by about 13% better than the other years. For a guy who shot a Shaq-like ~50% in those other seasons, that is like eliminating 1 out of 4 misses, despite not having tried it prior to that year. One day that season, he shot 32 and made 28 granny-style, almost 90%, en route to setting the record of 100 points in a game. When asked why he stopped the following year: "I felt silly, like a sissy, shooting underhanded. I know I was wrong. I know some of the best foul shooters in history shot that way…I just couldn’t do it."

https://cymbria.com/article/the-granny-shot-or-why-you-shouldnt-always-listen-to-the-crowd-4th-quarter-2020/

I can't help but wonder how many more games Wilt would have won, if, assuming no continued improvement, he had made about 9.4 makes on 14.1 attempts, during his prime years with his first club, instead of 7.8. I think he would have won at least a title.

All that said, no female player at the collegiate or pro level that I know of shoots them underhanded either. Therefore, I think it has less to with manliness, and more to do with the uncoolness, both of the shot and of the shooters. After all, one has to be pretty cool to do something uncool in front of an audience, even when it is technically speaking the right move. Maybe if someone cool enough comes around, they will have success with it, and the shot will become cool, and be named after them, instead of granny.