I can't speak for Reddit, but on HN the idea is not to place barriers in front of anyone who wants to join. If we demand that people jump through extra hoops, many will simply bail instead, including many who were inspired to jump in and comment because of something they saw on the site. That sort of spontaneous participation is precious. It keeps things fresh and makes HN more interesting. So although, yes, we incur a lot of costs (and pain) from spammers and trolls by being so open, the good things we gain are worth more.
To pick a couple of famous names, I doubt that Alan Kay or Peter Norvig would have bothered with HN if we'd placed up-front demands on them. Countless experts have shown up here over the years to create accounts and share firsthand about what they know. Here's a recent example that sticks in my mind: https://news.ycombinator.com/item?id=22007115. It's particularly common for project creators or article authors to show up and comment on their work. We don't want to do anything to make that less likely.
HN is fortunate in that we're not under pressure to monetize users, squeeze more engagement out of them, or optimize them in any way. The factors that drive companies into corralling their users and eking information out of them don't apply here. The only thing we need to do is have HN be interesting. An open community, assuming it can be persuaded not to destroy itself, is more likely to be interesting.