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So my question is: why do people often assume that trading competitions are only for whales?
Because whales have much more leverage and influence over the market in general than the average trader which only have a few thousands of dollars to trade. whales can set up red and green walls in order to manipulate the price of Bitcoin or any other alternative currency at their will, so they can accumulate or sell at the price range they see for suitable for their profits and in order to win the competition.
For example, a whale could set short orders on the futures market and then use part of their fortune to dump a massive number of crypto in the spot market, prompting the price to drop and his short orders to bring them a lot of money, those are movements which are impossible by someone who does not have millions or billions at their disposal.
I agree having the funds to shake the market is for the wealthy because they have the money to do so, if you are just small trader, you need to wait for the big player to be able to see where its going, this is very hard for small players, but if you can strategize and also know when to position or where to trade, that can compensate on unable to be able to move the market, also being experience will also help you, so there are some path you need to take to be able to bypass the lack of funds or having a small funds, but that is not easy from the start.