Because I didn’t want to hijack this thread…
Why is it that ladies buy their wedding gowns/bridesmaid dresses/prom dresses but the men just rent tuxes? Most women only wear them once where a guy can wear a tux multiple times.
First, some women do rent.
Second, many of the things women wear for weddings have to be fitted rather than coming off the rack.
Third, we have a cultural idea that men don’t care about weddings/prom and women do. This leads to things like men’s wedding outfits being rentable (because he doesn’t care about the tux) and women’s being preserved (because her wedding gown is important to her). The fact that many women and men do not fit neatly in these boxes does not destroy the appearance of their truth.
It’s not just weddings. Having helped my daughter through two prom seasons, it hit home how much girls spend on this stuff that boys don’t.
Also, the bride’s family needs to prove they aren’t poor/cheap; the groom doesn’t. It’s a rapidly outdated concept, but it framed the options available.
Times are changing…
My daughter got married a year ago. The groom and groomsmen provided their own clothing (grey jackets), because they are all recent college graduates and needed interview clothes, and the cost of renting a tux was about the same. The groom’s mom sewed them all matching ties.
I personally didn’t give a shit about what I wore to my wedding, although after renting a tux and finding the comfort, quality, and fit to be inferior I wished I had worn my own black suit. It struck me as one time use costume for an extra in a movie rather than an actual item of clothing.
For prom my date borrowed a dress from a friend and put me on the hook for a $150 tux rental plus dinner and flowers and she sure as hell tried for a limo.
I wish I had rented. I didn’t know it was an option at the time (maybe it wasn’t). I think it is becoming more popular now. I love my wedding dress and had grand ideas of having re-tailored for other formal occasions. I seem to completely lack said formal occasions however. Fitting in it now would require, um, effort. Months of effort. 
It’s just an archaic tradition. The wedding I just went to, the entire wedding party, bridesmaids and all, was wearing rented clothing.
Tuxedos do need to be fitted. While there is a chance that one the perfect size is available off-the-rack, for most that isn’t the case. Tuxedo rentals will be hemmed, tucked, etc. every time they are rented.
My WAG is that men, generally, are comfortable wearing a uniform, the only thing that may vary is the color. Women, again generally, want something more original and it would be more difficult to find enough other women to rent the same dress to make if financially feasible.
Same reason you don’t keep old Band-Aids.
Women’s fashions go out of style. No bride wants to wear last year’s wedding dress. Tuxes stay the same for centuries.
Because women are slaves to their emotions and form irrational attachments to inanimate objects that remind them of significant events… 
(Solely in the spirit of Hail Ant’s post) …and that’s why our husbands are still around.
That’s partially true. And even this year’s styles are so widely variable for women’s formal attire. On top of it, I think many women would be a bit skeeved at wearing a previously-worn dress for a formal event, whereas men don’t seem to have as much of a hangup about that.
Ask anyone who works in a thrift store or other place that collects used clothing; men tend to wear their clothes out, whereas women don’t.
Lots of people get married in their mother’s dress, few get married in their father’s tux.
My husband and I both already owned tuxes. Neither of us would fit into our mother’s dress.
I don’t think tuxes get altered to the same extent as a wedding dress might be, simply because the styles are so different. Remember, a tux is two separate pieces while a dress is a single garment. With two separate garments, you can order different sizes for the top and bottom if that’s how you’re built. The jackets come in different lengths. And neither the jacket nor the pants are expected to fit the way a dress does at a number of different points - if a guy has wide shoulders and a narrow waist, the alterations will probably not include taking in the jacket around the waist (if such a thing is even possible). I spent years, including the year I bought my wedding dress, needing different sizes on the top and bottom. In that case, you have to buy for the largest size and alter the rest. The dress that fit my chest was inches too big on my waist and hips , and because of my height, some alterations had to be done on top or else the neckline would have been much too low. Wedding dresses can be rented, but they generally only allow minor ,reversible alterations, so if I wanted to rent a wedding dress, I would have to either limit my choices to styles that were not fitted through the waist and hips and had a high neckline or accept that the dress wouldn’t fit properly.
I get half of the response, but are you a guy? (It’s the 21st Century.)
Yes, but I was already a guy in the 20th Century.
It’s because of brainwashing by the wedding-industrial complex!
I’d like to see some of the above responses changed to “Some women” because believe it or not, we don’t all think the same way. Some women dream of their wedding day from the time they’re little girls. Some of us elope over lunch in our uniforms, then go back to work for a court martial. (No, seriously, I did.)
The answers to Why? are as individual as the women being asked…