running your own open bar is actually pretty easy and affordable (compared to the other costs associated with weddings, that is) if you just buy your own alcohol and get a couple of volunteers to mix drinks. or you could just buy some champagne or wine, like paul said. or just buy a bunch of beer and keep it in a cooler, who cares.
most of this applies to relatively big weddings where a lot of cash is already being dropped and nobody is a recovering alcoholic so consider really tiny weddings between just a few friends or weddings where money is obviously a really massive, massive issue ruled out - but in a relatively medium-to-high volume wedding that is already a bit pricey, i honestly can't see any reason at all why some free alcohol shouldn't be included in there. it's the best way to thank your guests for their gifts, for their time and help, for making the trip, for taking the time off work and anything else going on in their lives, etc. to pretend that every single other person at your wedding is going to be as giddy and ecstatic and overjoyed and high on life as you are on your wedding day that they will be able to get through the entire day and night on sheer adrenaline alone is just not being realistic. of course they are happy for you and of course they are excited, but a wedding also takes all day and there's a lot of stuff that goes on. if you are neither the bride or groom it can even get to be a bit boring at times. consider the fact also that while you may know everybody at your wedding, they aren't necessarily all going to know each other. they might end up not knowing a LOT of people, even, and as a result may feel kind of awkward. having some drinks available is a really considerate way of saying thank you to everyone who's there and it helps people loosen up and meet each other and have fun. if you absolutely cannot for whatever reason have an open bar, you can maybe compromise by doing something like paying for 1-2 drink tickets per person to use at a cash bar. everyone does their own thing, but as much as it's your day and your wedding, you still have an obligation to be considerate and hospitable to the people who took the time out of their busy lives to come to your wedding, especially the guests who actually spent a lot of money on gifts or traveled a really, really long way to be there. you could also just send out invitations and accept the fact that a lot of people are going to decline to go to your wedding because they might be kind of bored or because it's a bit inconvenient, but getting a whole pile of "no" replies on a day that is really important to you really sucks so why do that when you can make your guests want to be there by just making it fun for them? like i said, i have been to a lot of weddings and the ones where some sort of alcohol is provided are infinitely more enjoyable, for pretty much everyone.