First chapters are important because they are the first impression agents and editors get of your work. These are busy people, they are culling a huge volume of submissions, and they're looking for an excuse to stop reading yours. From this fact, various myths about first chapters emerge.
For example, the idea that one must start with (or in the middle of) an action sequence is pervasive and wrong. It's a mutation of the adage that something must happen in those first pages (as opposed to back-story, world-building and other exposition).
Similarly, the idea that a first chapter can or should be markedly better-written or paced than the rest of the book is a canard. Every chapter should be your best work. Revising your first chapter should require much less time than making sure your middle section doesn't drag.
The hardest aspects of writing a novel are sustaining tension over an extended narrative, transitioning between plot-points, and providing necessary information and character development without bogging down in exposition. Compared to these challenges, first chapters are relatively easy.
The purpose of a first chapter is generally to introduce a primary character and a problem that will drive the plot. So, if you know who and what your book is about, the first chapter is halfway written.
Introducing a protagonist is easier than introducing a secondary character, since you have much less time and space to spend developing the secondaries, and they still need to seem three-dimensional. And introducing an initial motivating problem is much simpler than handling complications and twists later on.
The reason agents feel comfortable rejecting you based on a first chapter is because a flawed first chapter indicates a flawed manuscript. But just because those first pages can knock you out, it doesn't mean they can get you in. If you stick a polished first chapter on a poorly-written or plotted manuscript, that's not going to get you an agent. Your narrative should be polished and engaging throughout.