As a general rule, for a seat in the House of Representatives, it’s far, far easier to win as an incumbent. Unless you’ve actively pissed off your party, been generally an idiot or a criminal while in office, or had skeletons in your closet from your past come to light, you likely have your party’s support, and typically won’t see a serious challenge during your party primary.
And, relatively few congressional districts are particularly competitive these days – they’ve largely been gerrymandered to lean strongly to one party or the other. So, unless you’re in one of the relatively rare “purple” districts, you’re unlikely to lose the general election, either.
There was an article in yesterday’s Times about a Democratic woman in, I think, Michigan, who beat a sitting Republican two years ago. She said that a big reason she won was that the Trump cultists didn’t bother voting since their leader wasn’t on the ballot. He will of course be there in November and she was not terribly optimistic about her chance of holding on to her seat.
The heavy incumbency win rate is perhaps slightly skewed by the fact that, when an incumbent gets a strong sense that he/she is not going to get re-elected, they may simply choose to not run for re-election at all. There was a spate of Republicans in 2018 that chose not to stand for another term for this reason.
A lot of Republican Representatives retired in 2016 and again in 2018. Many of them figured they had a poor chance of winning again, or felt they didn’t have the energy to go up against a Tea Party or Trumpist primary challenger.
It’s certainly true that an incumbent is generally most vulnerable in their first reelection campaign. They’re still trying to figure out how to balance legislating and campaigning, they don’t have the seniority to bring back projects to their district or raise big donor money, and if they were elected in a “wave election” they may find themselves out of step with their district when the wave recedes. That said, it’s still better to be the incumbent with at least some name recognition, donor base and campaign apparatus in place for two years before the election.