I'm curious to know what your "guess" is based on because you are too specific about the altcoins and the amount of investment! Generally speaking it is hard to know these things unless you run a centralized exchange that people visit so that you can have a solid statistics and CEXes don't release such statistic for the public to know these things.

In any case, another possibility (assuming your stats are correct) is that you are looking at "traders" not "investors". When it comes to trading, altcoins are better than bitcoin although not the altcoins you listed here but altcoins that are getting pumped in that particular day so that you can buy them during the pump and sell them before their dump begins to make a decent profit in a short time.
Of course a lot of people trade using their bitcoins (not fiat) so that they can make profit on the amount of bitcoin they have. So your concern about people not having bitcoin is not correct in this scenario even though they are seemingly trading altcoins only.