Yes it's a very small % of the population, but it's actually a significant percent of the earnings of these platforms.

Just for illustration -- when I worked at Zynga they'd sell in-game purchases for over $10k USD for virtual items because there were people who just couldn't help themselves, and those "whales" were actually the bulk of the profit of the company.

That's why these platforms should have mandatory sane limits that can only be exempted with special circumstances.