While anything's possible, we have to look to the evidence. Mark doesn't present Jesus as a magic man, but rather emphasizes His preaching and authority.

[list][*] He is presented in Mark 1.1-3 as the Messiah, the Son of God, and the Lord (all identity labels). No "magic" is implied from the starting line.
[*] Mk. 1.8. Then Jesus is predicted to be one who will baptize with the Holy Spirit. Again, not a doer of magic, but as one who brings the Spirit.
[*] The baptism again has identity markers. This is Mark's concern.
[*] In public ministry, His first image is that of a preacher and teacher (1.14-15), not a miracle worker let alone a magician.
[*] When He calls disciples, the invitation is to follow His person (1.17). No miracles ("magic") have yet been done.
[*] When He does His first miracle in Mark (1.21-28), Mark's emphasis is on His identity (Holy One of God), not on the miracle itself. Even the people's reaction is the authority of His teaching (1.27)—He orders and spirits obey.
[*] Even in the healing many of 1.32-34, Mark breezes through the miracles; his emphasis is Jesus's authority ("He would not let the demons speak").[/list]

Without belaboring the point, this emphasis of Mark continues throughout the book. His concern is Jesus's identity and authority, not his "magical" capabilities. When he mentions the miracles, the point is Jesus's authority and identity, not His magical power.

In the feeding of the 5000, the miracle part is considerably understated (Mk. 6.41-42: "Then he gave them to his disciples to distribute to the people. He also divided the two fish among them all. They all ate and were satisfied."). Mark's point is Jesus's compassion (they are sheep without a shepherd, v. 34). The pericope is told to convince us of Jesus's identity and authority.

So also in the feeding of the 4000. Jesus's compassion is highlighted (8.2). Again the miracle itself is greatly understated. They are both (both feedings) allusions to the Exodus, and therefore his identity and His authority. Mark follows this section with a challenge by the Pharisees, an allusion to how the Israelites tested God.

Mark is not telling stories of a magic man, but rather that Jesus is the Messiah, the Son of God, and the Lord.

Why can't it be as simple as the author of Mark wanted to reinforce the idea that Jesus had magical powers so he repeated the story?

William Lane (NICNT, Mark, p. 269) says that Mark has written the two events to be similar and also parallel.

6.31-44 Feeding of the Multitude 8.1-9
6.45-56 Crossing of the Sea and Landing 8.10
7.1-23 Conflict with the Pharisees 8.11-13
7.24-30 Conversation about Bread 8.14-21
7.31-36 Healing 8.22-26
7.37 Confession of Faith 8.27-30

With the repetition of sequence and events and the references to the exodus, Mark’s point may be that the message of the gospel for the Gentiles is the same as it is for the Jews. It’s an appropriate segue after the story of the faith of the Syrophoenician woman.

Jesus is in the Decapolis, an area dominated by Gentiles. The new people of God is Jews and Gentiles together. He has compassion for all people just as he has had for the Jews.

Feeding of the 4,000 ... what led Mark to think this was definitely a separate event from feeding the 5,000?

The feeding of the 4,000 appears only in Mark and Matthew (presumably using Mark as a source). Mark and Matthew both also have the better known 5,000 story earlier in the narrative.

A simple glance makes it seem like these are almost certainly just variants of the exact same story with slightly changed numerical details.

What do scholars think could have been the reason Mark considered these to be definitively separate events? And is it thought that Luke excluded it because he thought they were probably variants on the same event?