Understanding the Use of `default` for Undefined Environment Variables

const { PG_HOST, PG_PORT, PG_DATABASE, PG_USERNAME, PG_PASSWORD, PG_USE_SSL } =
  Schema.parseSync(
    Schema.struct({
      PG_HOST: Schema.string.pipe(Schema.default('db')),
      PG_PORT: Schema.int()(Schema.NumberFromString).pipe(Schema.default(5432)),
      PG_DATABASE: Schema.string.pipe(Schema.default('postgres')),
      PG_USERNAME: Schema.string.pipe(Schema.default('postgres')),
      PG_PASSWORD: Schema.string.pipe(Schema.default('topsecret')),
      PG_USE_SSL: BooleanFromString.pipe(Schema.default(true)),
    })
  )(process.env);

core:  FAIL  src/lib/core.test.ts [ src/lib/core.test.ts ]
core: Error: error(s) found
core: └─ ["PG_HOST"]
core:    └─ is missing
core:  ❯ ../../node_modules/@effect/schema/Parser/dist/effect-schema-Parser.esm.js:33:13
core:      31| const getEffect = (ast, isDecoding) => {
core:      32|   const parser = goMemo(ast, isDecoding);
core:      33|   return (input, options) => parser(input, {
core:        |             ^
core:      34|     ...options,
core:      35|     isEffectAllowed: true
core:  ❯ src/lib/core.test.ts:27:3