The answer is two-fold.
They don't increase human acceptance of the free gift He offers us. The Bible shows this bizarre phenomenon throughout history. Pharaoh had a front-seat to the power of God and remained unimpressed, a heart like granite dead in his chest. Even the Israelites themselves, the very generation whose freedom from slavery was won by these miracles ... who passed between the great walls of water along the bottom of the Red Sea ... even they fell away after a time even as the manna dissolved on their tongues.
"They forgot His deeds
And His miracles that He had shown them ...
Yet they tempted and rebelled against the Most High God
And did not keep His testimonies,"
- Psalm 78: 11 and 56
Even when Jesus walked the earth, performing very visible miracles before men ... they didn't believe. He cursed several Jewish cities where He performed His miracles ... people who should have been watching and waiting for the Messiah. Imagine, if you will, you are asked to pick someone up at the airport. You are given a description of them ... what they look like, you have an idea what they might be talking about because you know the one who is sending them to you. But on your way perhaps you begin to forget, or you don't really trust the person who sent you to pick them up and you start to wonder if their description was accurate? So you go to the airport and wait ... and wait ... and wait. Your very life, eternally, depends on finding this person! But because of your distrust of the one who sent him ... you miss him. He walks right past you, saying the things you should have expected him to say ... fitting the many descriptions you were given ... doing things only he could do. And you missed it.
That's what happened to the Israelites in these Jewish cities. Jesus walked right amongst them, and even though He performed miraculous things, they didn't believe.
"Then He began to denounce the cities in which most of His miracles were done, because they did not repent."
- Matthew 11:20
Perhaps, like the Jews of His day, we're flat out missing out on miracles happening all around us. If we have the scales removed from our eyes and begin each day with anticipation, perhaps we will begin to see that we are surrounded by miracles. Small and large impossibilities, improbabilities ... favor poured out on our heads like blessings from the Lord.
Dare I suggest that the more miracles we recognize, the more we will see? Instead of demanding a miracle in a time of great need, if we are in the practice of recognizing the small miracles He showers us with on a daily basis ... if our hearts are ready to believe ... perhaps we are more likely to see more? I only suggest this because Jesus didn't perform many miracles in His own hometown. Why not?
"And He did not do many miracles there because of their unbelief."
- Matthew 13:58
Maybe the reason we don't see many miracles these days is because we just don't believe in them anymore ... maybe they're staring us in the face and we're staring back with faithless, blind eyes.