Building a community focused on something isn't hard, you grab a few people you know with shared interest, and they grab and invite their friends. Small game development communities everywhere thrive, I've never been a part of one that wasn't doing well.
Building a large community not focused on anything is much harder.
Dead subreddits, dead forums and abandoned meetups might disagree with that assessment. Building a community is far from easy and there's a lot of effort involved to keep it going as an organizer or moderator. Setting it up initially is easy, keeping it going for many years is hard.
I've set up, moderated forums, irc servers and organized a regular meetup for many years and I've seen that all the time.
Well you said it yourself. the beginning and short term is easy.
maintaing is very, very, hard. As a website, you fight for attention against decades of algorithms, SEOS, trillions of dollars of ad revenue, and you still gotta be picky to not pick up trolls. IRL, you're fighting for time in a person's schedule to travel and incentivizing them to keep coming in person while people are more overworked and less compensated than ever. Even the most benevolent, well mattered communities that attract high quality participants will decay naturally. So maintaning is a never ending, thankless job.
Building a large community not focused on anything is much harder.