How can someone reach Challenger 2000 lp relatively close to the season start and then not make it to 3000 lp or more easily? Why are people seemingly stuck around 2000 lp?

These answers aren't really correct. You can look at the the players at the top of the ladder and almost all of them will be roughly around +-20.

The reason they don't get higher is simply because they are winning just as much as they're losing (50 percent winrate), because they aren't maintaning a 65% WR when they're at rank 1. They might have been when facing master, gm but not at the top.

Part of this is also because if they are actually that good (eg. Say they are 4000 mmr when the other 9 players are closer to 3000), then the teams will be balanced such that each team has a 50% chance to win (ie. The 4000 mmr player will have the lower players on their team). If the player is good enough, they'll be able to maintain that higher winrate and start to push the boundary (similar to what srtty was doing last season on NA)

Another interesting case study would be if you were to put a soloq god like showmaker or canyon in a much weaker server, you'd see the lp ceiling get smashed, because they can carry the top of challenger regardless of who their teammates are.