God has revealed Himself to us. He has given to us special revelation (Luke 16:29-31). He has revealed Himself to us through creation (Romans 1:20). He has revealed Himself to us through the Person of Jesus Christ (Hebrews 1:1-3). He speaks to us daily through the Holy Scriptures.
If miracles happened frequently, then they would cease to be miracles. They could no longer be used for the specific purposes that God ordains. Miracles would be ordinary events which no longer capture our attention.
God has not manifested Himself directly to human beings throughout most of history. Practically every person of faith has not had such an encounter with Him while on this earth. What makes us so special that things should be different? We should not be shocked or embarrassed that God does not speak audibly to us right now.
The question should not so much be why God does not reveal Himself to us in a more obvious way, but when is enough evidence enough? There are people who would not be satisfied even if He spoke audibly to them or brought someone back from the dead.