Their website makes a lot about participants are supporting charities. Doubtless there will be some who think if they don't win, the money goes to good causes (no mention of the 20% cut of the profits that go to Omaze, a US for-profit organisation). There are subscription plans where those that sign up are automatically allocated places in the draw, and gift cards, meaning the recipient can choose which property they buy tickets for/ charity they support. Some people must be signed up for these.I would imagine that sufficient number of people are gullible enough to fall for their slick advertising so that they can make their money from the sum total of all the entries to each prize draw. If they didn't, they wouldn't be doing it.
One of the draws for Teenage Cancer Trust made £800,000 (after costs) from their 80% share, so Omaze made $200,000. They are doing alright! As you say, it must be working!!
Source https://omaze.co.uk/pages/teenage-cancer-trust