I have two 5th generation 4Runners and a Tundra.
The Tundra and 4Runner I drive both have this problem. Both started around 20,000 miles. This obviously makes me think it is something about my driving style (holding the top center) that causes the problem.
Being said...
The top and bottom of the steering wheel is made from vinyl and the sides are real leather. On mine the black finish is rubbed off and showing the sub white uncolored vinyl at the top. Toyotas are great cars and I have driven them for years. I think it is kind of cheap of them that somehow it makes financial sense to make the top and the bottom of the steering wheel out of vinyl and not just make the whole wheel from leather. I had a 2003 4Runner and the whole wheel was leather. It did fine for the 13 years I had it. Mixing vinyl and leather on the seats as a cost saving measure has been going on for years. Some bean counter at Toyota figured that the majority of people must hold the wheel at 2 and 10 or 3 and 9 and that they can get away with making the top and bottom from vinyl!

Toyota did warranty my 4Runner steering wheel but I could not ask to warranty the Tundra wheel because I had put it on myself. (Same exact Toyota factory wheel).
They did not change out the 4Runner wheel but instead gave a decent amount of service credit at the dealer as compensation. I accepted that because I figured a new wheel would just do it again.
I have had reasonable luck repainting it/re-dying it with kiwi leather dye. It comes in a shoe polish foam tip. I taped it off and put a drop cloth so as not to drip in the wrong places. It dries surprisingly evenly for a foam applicator. The kiwi does wear off over time but takes quite a while....it does not come off on your hand. They use the same stuff to dye purses and shoes. It is a little bit shiny but dulls with use.
There are after market wheels that have leather accent sides and plastic top and bottom that are as little as $200. Its not that hard to switch it out with a $15 wheel puller, some YouTube videos and some courage. Once mine gets very bad I am going that route. Or maybe you could get a shop to do it for $150 or so.
I think if the whole wheel was leather it would not have had this problem. I do agree with a lot of others that Toyota picked the wrong place to cheap out. It is right in your face everyday!

For what is worth.