You don't have to pay to have a minecraft server up.
Hosting a server is simple. It just depends on the fact that you have a computer connected to the internet which can act as the host, and the software for hosting the server.
When hosting servers however, if you want it to be a dedicated server (like we have the ability to do in Blockland), you need to keep your server up all the time. Which means having your computer on all the time. Which will cost money as you use electricity.
Most people also can't keep servers up all the time due to them being family-owned and an entire family wanting to use it.
When people pay for a Minecraft server that is them giving money to someone else who owns a server-computer to host your server for them.
It's the exact same way as how you can pay to host a server on Blockland, if you pay server-hosters, like Kalphiter's server hosting or the one RTB is launching.
You could easily host Minecraft servers without paying anyone, but the chances are your server will likely have to shut down multipe times for whatever reasons, in the same way that most people don't keep Blockland servers up all the time.
Plus, server-hosting companies usually have computers specifically designed for hosting servers. They're not using their home laptop.