Whenever people bring this up, they neglect to consider that there are factors beyond sitting comfort and protection.
This also depends on wallet and pocket size, what else you're carrying, and even style of pants.
There's the immediate question of whether the wallet will even fit in the front pocket - even large pockets sometimes have weirdly-angled openings that make dealing with a wallet difficult. Back pockets tend to be less narrow and virtually always have a simple horizontal opening.
Men tend to carry a lot of items in pockets - you probably have keys, maybe a pen, and, increasingly often, a bulky smartphone. All of those things are considerably less comfortable to sit on than a fairly flat object that comforms to the shape of your ass over time.
And some styles of pants just look stupid with a wallet in the front pocket. If you're wearing tighter pants, for instance, you're going to have a big blocky bulge in your leg.
Also, getting pick-pocketed is fairly rare in most of the United States. Even in places where it's more common, it's more tourists than natives and you learn what to watch out for anyway. And if you're going somewhere sketchy, you always have the option of just switching it to a front pocket. Most people probably spend most of their time in places that they're not so worried about being pick-pocketed though.