A cPanel license is dirt cheap in comparison to the massive investment of time and effort required to build a custom panel. A large proportion of most businesses expenses are caused by staff, like salaries, taxes and insurance. Building a panel takes effort, effort takes people, and people cost money.
But most of the biggest free hosting providers in the world use a custom panel. That’s because a custom panel allows you to design the system from the ground up to work well for free hosting, and implement security features and cost saving measures which are necessary to provide free hosting sustainably.
There are free hosting providers who use cPanel. But most of them have signup restrictions, country restrictions, or are fly-by-night hosts with great ambitions today who will be gone tomorrow.
And last summer, cPanel changed their billing, so it’s no longer a flat fee per server. Hosting providers also need to pay them at least $0.10 per account per month now. Which is no problem for a premium account of $5 per month, but definitely a problem for free hosting.