The Question is Can we use present tense with subjunctive mood?
The Poster commented; As far as I know, present tense cannot be used with subjunctive mood, so I'm asking this question.
Regarding your first example
Why isn't there even a little piece of evidence such as crater anywhere on Earth?
The answer is No we cannot. To explain myself more, we cannot mix tense's. To explain I suggest your question would have been better phrased as Why has the writer used both subjunctive mood and present tense in the same sentence. The fact is the writer has not mixed tenses, what he has done is combined two clauses which could have been written as two separate sentences as each clause is complete. The ability to use two different tenses in the same sentence thus stems from the fact that the two clauses are complete and could stand alone.
If such a huge meteor had really crashed into the Earth. Why isn't there even a little piece of evidence such as crater anywhere on Earth?
Now let's consider your second example.
If he were really a famous singer, why can't he sing very well?
"If he were really" Although this is fairly common speech, or it is in my part of the world. If he were really is a phrase meaning He's not a Famous singer. Then the writer goes on to qualify this by asking the question why can't he sing very well? The meaning of the complete sentence being He's not a Famous singer you can know he is not, because he cannot sing well.
The point being that If he were really a famous singer and Why can't he sing very well? cannot be used as stand alone sentences and keep the same meaning. They would become two statements (statement, plus statement question) rather than one (interdependent) suggestive sentence.
If he really "is" a famous singer, why can't he sing very well? is the Sentence we should be using and that would not mix tense's.
really adverb (NOT IMAGINARY)Cambridge English dictionary