Perhaps He doesn't want to. Why should he? Just because some atheists who mock God expect Him to appear?
He answered, "A wicked and adulterous generation asks for a sign! But none will be given it except the sign of the prophet Jonah." (Matthew 12:39)
I find it funny how atheists claim to love science, yet, science has discovered many things that we can not see with our own eyes, from subatomic particles, to bacteria, to distant galaxies. None of those things have ever appeared to us. Why should we even expect God to appear to us.