Answer

Anyone can understand this on their own.
First, there are many Haredim who leave as well, but the phenomenon isn’t talked about and isn’t on the table. Second, there are lots of hollow Haredim who don’t dare leave religion, but inside they are empty of faith. Third, when you allow fewer options to be open to people—that is, they are less familiar with the alternatives (and are fed empty slogans rather than full and open information)—and when very heavy social and spiritual pressure is applied and people are given no real way to exist outside the shtibl (with no livelihood options, and pressure on the family), the result is that fewer leave. The phenomenon of the forced converts is proof enough.