Why is this an issue?

Nested code - blocks of code inside blocks of code - is eventually necessary, but increases complexity. This is why keeping the code as flat as possible, by avoiding unnecessary nesting, is considered a good practice.

Merging if statements when possible will decrease the nesting of the code and improve its readability.

Code like

if (condition1) {
  if (condition2) {             // Noncompliant
    ...
  }
}

Is more readable as

if (condition1 && condition2) {
  ...
}