I agree that a child of a datalist element should not block the form
submission. However, I'm wondering why do we care about this particular
edge case when there are a lot of situations where an element can be
invalid without any possible action from the user.

There is this backward-compatibility friendly use of datalist that requires to have a select inside a datalist like:
<input list='d'>
<datalist id='d'>
  <select>
    <option>Option1</option>
  </select>
</datalist>

However, this use case doesn't justify barring from constraint validation because the only situation where that would be needed is if <select> is required and the first option is making select suffering from value missing.
However, using required in a backward-compatibility path would be wrong and using required wouldn't follow how lists are working (ie. you don't _have_ to select something).

Is there any other reasons I'm missing?