Actually something is gained as buying too early means that you first have to dig yourself out of the hole before the account turns from red to black. Waiting pays, as long as one doesn't wait too long.
I've seen at least 18 of one colour happen consecutively on a roulette wheel in a casino. When you consider all the roulette wheels in all casinos everywhere, spinning every second of the day, the probability of this happening somewhere is quite high, 90%+.
Assume that the chance of landing on black is 0.5, or 50%. (It's actually a bit lower because of the null.) Then the probability of 18 consecutive blacks would be 0.5^18, or ~0.000004. Very, very remote odds. Either you witnessed an extremely unusual event, or the wheel was flawed. As for your comment about all casinos everywhere, the same can be said about lotteries. But that still doesn't improve the odds for any individual lottery ticket buyer. So I'm not quite sure what your point is.
I believe the rationale is that luck comes in bunches and going against a trend jinxes it or the gambler. Most gamblers are a superstitious lot. My Kelly fraction for a negative-expectation game is zero so that is what I bet: zero.