Why is date-only string treated as UTC instead of local in JS [closed]

For example, when I pass a full date & time string to Date()

new Date('2020-01-01 00:00:00').toISOString()
// '2019-12-31T17:00:00.000Z'

it treats the input as local time.

However, when I pass just a date string

new Date('2020-01-01').toISOString()
// '2020-01-01T00:00:00.000Z'

it treats the input as UTC time.

Why is this behavior the default?