There are online conferences, but they're very different from the in-person ones. Here are some properties of in-person conferences that are difficult to replace.

* Some talks are long, intricate stories, and you get much more out of them by paying close attention. (David Beazley comes to mind.) From experience, I can say that I'm far more likely to divide my attention at home than in an in-person talk, and I'm not unusual here.

* Sometimes, there's no talk that grabs you in a time slot, but that can lead to pleasant surprises. This is especially effective if you intentionally choose topics that are unfamiliar, then let the physicality of the room force you to pay attention (because it would be rude not to).

* Running into people randomly in the hall makes for serendipitous conversations that wouldn't have happened otherwise. People get jobs this way; open source projects start this way. The global Code Retreat movement started this way due to a conversation at CodeMash years ago. (That conversation began by us talking about the ethics of the meat in the sandwiches.)

* If there are people who you put on a pedestal as untouchable divine entities, meeting them will usually fix that. I don't mean that they don't deserve respect, but simply that everyone is human. You might be appalled to learn that your favorite programming tool's author likes music that you find repugnant. This is valuable; we're people. You'll be less likely to appeal to authority unthinkingly if you've been appalled by authorities' taste in music.

* It's vulgar to put this so bluntly, but a lot of power is brokered in person at conferences. Not through mysterious back channel conspiracies, but through normal human interactions. If I've met you in person, I'm more likely to trust you; you're more likely to come to mind when someone asks "do you know anyone who can do x?"; etc. Anyone who doesn't participate is at a disadvantage here (and many surely don't care, which is fine).

(Source: I've spoken at about 40 conferences, give or take, and attended many without speaking as well; I've made a lot of friends who do the same; I shot an unpublished documentary that was sort of about this; and I have repugnant taste in music.)