You should read Alexis King's articles on how this is achieved. One of the most imporant aspects of modern parser combinator libraries is that a very small composable core can be exhaustively tested (importantly: with both hand written and random cases) to provide a reliable kernel for parser combinators.

Given that those cited parsers were built from the ground up to mesh with an effective and expressive type system like Haskell's, this problem was significantly reduced. The downside, of course, is that it became quite opinionated.

There is no magic want to wave, but certainly it's easier to work on a common minimal core of components than work on custom ad hoc validators. Lax parsers are not an inevitable outcome; they're a decision propagated by decades of bad opinions being taken as gospel.