In addition to what Sodium said, incumbents have resources from their office to use, plus they don’t have to deal with bureaucracies other candidates have to go through.

For example, an incumbent meets on a regular basis with the press as part of their position. When campaign season rolls around, they already know the media organizations used to spread their platform. A newer candidate doesn’t have access to those same channels. Then stuff like Bush raising “terrorism possibilities” during his 2004 campaign against John Kerry, something Kerry couldn’t do.

As for the bureaucratic portion, a newer candidate might have to do something like collect signatures in order to run. Their campaign has to spend time going door to door before they’re even on the ballot, just to get on the ballot. The incumbent, however, is automatically on the ballot so their campaign can focus on things like donors. You see this a lot in Congress with party leaders, such as Nancy Pelosi and Mitch McConnell. In order to primary Pelosi, a person has to go through a lengthy process in her district to even attempt removing her, then actually running a campaign to remove her. Meanwhile, Pelosi gets to talk to people she’s met through her office, stacking up on donors, endorsements, favors, etc.