Because they aren't having to pay for hosting.
The only way to replace reddit is with a distributed system like aether: https://getaether.net/
Or if you absolutely want a centralized system, something community run like ao3: https://en.wikipedia.org/wiki/Archive_of_Our_Own
But given the absolute hostility and hate _users_ of reddit give those two sites for not banning everything they find offensive a site like reddit is just not possible any more.