Most anime, including Death Note, are adapted from manga (basically the Japanese equivalent of comic books). This leads to a lot of internal narration and recapping for two reasons:

1. Manga, like comic books, are a quite limited medium. In a lot of comic books, particularly older ones, you will see characters internally-monologuing or repeating unnecessary information to make it clear what's happening in the panel, and what the characters are thinking. When manga get adapted into anime, these monologues tend to get included because...
2. Manga are often adapted at a rate of one or two chapters per episode, but if there isn't enough material for a full twenty-minute episode, they're forced to pad it out with filler. This can include stretched-out camera angles, but also egregious flashbacks/recaps and lengthy monologues. My Hero Academia, IIRC, is adapted at a rate of half a manga chapter per episode; take a shot every time Deku flashes back to something you've already seen, and you'll be dead by the end of Series 1.

EDIT: As you noted, not all anime do this. Some adapt more chapters per episode - comedies and slice-of-life shows do this in particular, as they don't have to worry about pacing. Sometimes new material will be inserted to fill in the gaps, including entire arcs full of filler if they've caught up to where the manga is, but said filler might also contain large amounts of monologues a) so it matches the rest of the series and b) to give the manga an easier time catching up.

Most anime, including Death Note, are adapted from manga (basically the Japanese equivalent of comic books). This leads to a lot of internal narration and recapping for two reasons:

1. Manga, like comic books, are a quite limited medium. In a lot of comic books, particularly older ones, you will see characters internally-monologuing or repeating unnecessary information to make it clear what's happening in the panel, and what the characters are thinking. When manga get adapted into anime, these monologues tend to get included because...
2. Manga are often adapted at a rate of one or two chapters per episode, but if there isn't enough material for a full twenty-minute episode, they're forced to pad it out with filler. This can include stretched-out camera angles, but also egregious flashbacks/recaps and lengthy monologues. My Hero Academia, IIRC, is adapted at a rate of half a manga chapter per episode; take a shot every time Deku flashes back to something you've already seen, and you'll be dead by the end of Series 1.

As you noted, not all anime do this. Some adapt more chapters per episode - comedies and slice-of-life shows do this in particular, as they don't have to worry about pacing. Sometimes new material will be inserted to fill in the gaps, including entire arcs full of filler if they've caught up to where the manga is, but said filler might also contain large amounts of monologues a) so it matches the rest of the series and b) to give the manga an easier time catching up.

Most anime, including Death Note, are adapted from manga (basically the Japanese equivalent of comic books). This leads to a lot of internal narration and recapping for two reasons:

1. Manga, like comic books, are a quite limited medium. In a lot of comic books, particularly older ones, you will see characters internally-monologuing or repeating unnecessary information to make it clear what's happening in the panel, and what the characters are thinking. When manga get adapted into anime, these monologues tend to get included because...
2. Manga are often adapted at a rate of one chapter per episode, but there usually isn't enough material for a full twenty-minute episode, so they're forced to pad it out with filler. This can include stretched-out camera angles, but also egregious flashbacks/recaps and lengthy monologues. My Hero Academia, IIRC, is adapted at a rate of half a manga chapter per episode; take a shot every time Deku flashes back to something you've already seen, and you'll be dead by the end of Series 1.

Most anime, including Death Note, are adapted from manga (basically the Japanese equivalent of comic books). This leads to a lot of internal narration and recapping for two reasons:

1. Manga, like comic books, are a quite limited medium. In a lot of comic books, particularly older ones, you will see characters internally-monologuing or repeating unnecessary information to make it clear what's happening in the panel, and what the characters are thinking. When manga get adapted into anime, these monologues tend to get included because...
2. Manga are often adapted at a rate of one or two chapters per episode, but if there isn't enough material for a full twenty-minute episode, they're forced to pad it out with filler. This can include stretched-out camera angles, but also egregious flashbacks/recaps and lengthy monologues. My Hero Academia, IIRC, is adapted at a rate of half a manga chapter per episode; take a shot every time Deku flashes back to something you've already seen, and you'll be dead by the end of Series 1.

EDIT: As you noted, not all anime do this. Some adapt more chapters per episode - comedies and slice-of-life shows do this in particular, as they don't have to worry about pacing. Sometimes new material will be inserted to fill in the gaps, including entire arcs full of filler if they've caught up to where the manga is, but said filler might also contain large amounts of monologues a) so it matches the rest of the series and b) to give the manga an easier time catching up.